Last offseason, the D-backs were known to be one of the teams talking to the Orioles about Manny Machado, for some obvious reasons. This summer, Arizona is known to once again be one of the teams in on Machado, for many of the same obvious reasons.
Baltimore didn't end up moving Machado last offseason, obviously, as the club hoped it might compete this year. That didn't work out, so the last-place O's have no choice other than to trade Machado by the end of the month. He'll almost certainly be the biggest game changer available, one of the rare players -- perhaps the only player this year -- capable of single-handedly changing a postseason race.
That means there's going to be more than a few teams interested, and you could certainly make a case for the Indians or the Dodgers or the Brewers or the Cubs or a half-dozen others to go for it. We're going to use this space to advocate for the D-backs. There's not a more perfect fit of player and need.
Because there's no time like the present for the D-backs
Since the D-backs endured five straight seasons without topping .500 before last year's surprising postseason appearance, they may seem like the new kids on the block, but they're not exactly a young team. Arizona has the second-oldest lineup in the National League behind San Francisco, and the third-oldest pitching staff. Zack Greinke turns 35 in October, and Patrick Corbin and A.J. Pollock are in the final years of their contracts. (Plus, Paul Goldschmidt can be a free agent after 2019.)

That's not to say that if the D-backs don't win this year that they can't going forward; it's to say that there are times to hug your prospects and there are times to be aggressive. For a first place team trying to fend off a Dodgers club that may for once be restrained in spending, now is its time. We saw how much Arizona benefited last year when it picked upJ.D. Martinez, and this is the same idea.
So what do the D-backs do? Sure, they could use a reliever, but they have the Majors' lowest bullpen ERA. They could use a starter, but it'd be hard to improve upon a top three of Greinke, Corbin and a healthy Robbie Ray. The weakness is clear: They need a bat to improve an offense that's 23rd and 24th in a pair of advanced metrics, and last in the NL in batting average.
Because the NL West is still very much in question
It's true that the D-backs are in first place. It's true that, with the exception of a week-plus span at the end of May, they've been atop the division all season long. What's not true is that things are wrapped up. Since May 16, when the Dodgers bottomed out with a sixth straight loss to Cincinnati and Miami, Los Angeles has had the NL's best record, 28-13.  
In fact, despite the lead, our postseason projections actually see the Dodgers as the favorites to take the West, giving Los Angeles 63-percent odds to Arizona's 29 percent. (It doesn't help that the D-backs were just swept at home by the Giants.) That doesn't guarantee anything about the future; it does show you that there's no reason to take the foot off the gas pedal now.
Because adding a bat could fix some very serious platoon issues
Even with Goldschmidt's resurgence and Daniel Descalso's breakout season, the D-backs still have an offense problem, an infield problem and a left-side platoon problem. (Their total line of .228/.305/.396 is basically even with the White Sox and Tigers in the bottom quarter of the Majors, which should tell you something.)
First of all, there's the fact that Nick Ahmed, Ketel Marte and Jake Lamb simply aren't hitting enough. In fact, Arizona is the only team that has given at least 150 plate appearances to three infielders who all have a below-average OPS+. This isn't unexpected for Ahmed and Marte, who are hitting at or near their career norms, and Marte's brief hot streak had stopped over the past week, even before he injured his hamstring this weekend.
Lamb missed a chunk of the year with a left shoulder injury, so it's fair to expect more from him -- against righties, that is. Lamb has proven himself completely incapable of hitting lefties in his career, with a line of .163/.272/.300 against southpaws. 

In fact, let's use that truth to propose a unique job share, one that wouldn't just allow the D-backs to add an elite bat to their lineup, but one that would make their existing players look like the best versions of themselves: a split platoon. Since Machado has experience at both short and third, he could switch spots depending on the handedness of the opposing pitcher.
Just look at how it would work for the two left-side spots ...
D-backs' third basemen
vs. LHP: .211/.284/.333 (25th in MLB)
vs. RHP: .236/.335/.426 (15th in MLB)
D-backs' shortstops
vs. LHP: .296/.361/.551 (3rd in MLB)
vs RHP: .209/.262/.386 (25th in MLB)
Imagine taking all of those third-base plate appearances against lefties away from Lamb (as well as Descalso and Deven Marrero) and giving them to Machado, who hits lefties well. (.275./327/.495 against them this year, .281/.331/.464 in his career.)
Imagine, also, taking all of those poor shortstop plate appearances against righties away from Ahmed and Marte and giving them to Machado, who smashes righties. (He's hitting .325/.397/.592 so far in 2018, .283/.334/.497 for his career.) In addition to adding an elite bat in Machado, this makes two spots better and it helps protect Arizona's No. 2 defense, since Machado rates better at third than he does at short.

With Pollock expected to return from injury by the All-Star break, Goldschmidt hitting as well as he has ever hit and David Peralta hitting at an All-Star level, the D-backs already will have three strong lineup spots going forward. Machado would make it five, one by himself, and one by extension on the left side. (You can dream on Steven Souza Jr., who has scuffled through a disappointing and injury-plagued debut, though that's getting ahead of ourselves.)
Because if the D-backs get Machado, the rest of the NL does not
You know who else needs a shortstop? The Brewers, Dodgers, Phillies, maybe the Cardinals and Braves, perhaps even the Cubs, if they choose to make a big splash. Other than Machado, they'll get to choose between Eduardo Escobar, who is having a good year, and Jose Iglesias and Adeiny Hechavarria, who are not.
You know who needs a third baseman? The Phillies, Cardinals and Braves, at the very least, and the options are suddenly limited: Josh Donaldson isn't healthy, and Mike Moustakas is a fraction as valuable as Machado is.
Those are the teams the D-backs need to beat to get to the postseason, and to beat if they're in the postseason. First, they need to beat them for Machado, if only so their competitors don't have him.
While Ahmed is a strong defender, the fact is that Arizona badly needs a bat, and its shortstops are projected to be the second worst in the Majors from this point forward.
This isn't the time to wait. It's the time to go for it. It's what first-place teams do.