After a flurry of rumors Monday night, the Chicago Cubs and Arizona Diamondbacks completed a trade on Tuesday. Catcher Miguel Montero, 31, became a Cub, and Chicago sent minor-league pitchers Jeferson Mejia and Zach Godley in return. The deal creates payroll flexibility for Arizona, helping them confidently address the holes elsewhere on their roster. (After spending $68.5 million for a six-year commitment to Yasmany Tomas, they needed the relief.) It gives Chicago a left-handed bat who can have a positive impact in the lower half of the order, and a huge defensive upgrade behind the plate, and a structural flexibility they needed badly.

Montero is one of the dozen best framers among all big-league catchers. He turns balls into strikes, and secures the strikes his pitchers throw to the edges of the zone, very well, and that skill has proven to be the most valuable one a catcher can have, defensively. Few players can make as big a difference as a good catcher, because they have the opportunity to make other players—in this case, pitchers, specifically—better. Montero falls into that bin. Welington Castillo, whose role as the Cubs’ starter Montero takes over, was one of the worst pitch-framers (and therefore, a poor defender overall) of the past two seasons. Chicago gets a big boost, then, even if Montero is unable to shake off the consecutive seasons of offensive struggles that were his 2013 and 2014.

I believe the Cubs will make out even better, because Montero strikes me as a candidate to recover nicely and add length to a potent lineup in the coming years. Rather than retrace my steps on that issue, though, I’ll simply refer you to this piece, from the immediate aftermath of the Russell Martin signing, where I lay out the fit between Montero, the Cubs and Joe Maddon. Right now, to put a new and different spin on the matter, I want to discuss the structural flexibility the Cubs gain by adding Montero. It might be as valuable as the on-field, in-game, talent-based value Montero can offer.

First of all, consider the balance of the Cubs’ lineup. Anthony Rizzo was the Cubs’ best hitter and an All-Star-caliber first baseman in 2014, but until they added Montero, Chicago did not have another projected long-term left-handed bat. Jorge Soler, Javier Baez, Starlin Castro, Kris Bryant, Addison Russell and Albert Almora all bat right-handed. By adding a left-handed batter who hits right-handed pitching well, and by doing so at a position where it is inherently difficult to find left-handed batters, the Cubs helped ease any concern about imbalance. Bringing Bryant up, for instance, will require moving either Luis Valbuena or Chris Coghlan—two left-handed hitters who posted a composite .787 OPS in 2014. That would have been a source of consternation, if not an actual problem, before this trade. It’s now much more palatable.

That’s just one dimension in which the trade gives the team flexibility, though. This also allows them to bring 2014 first-round draft pick Kyle Schwarber slowly, as a catcher, or to move him out from behind the plate and get him to the Majors very quickly. It also creates an opportunity to trade Castillo, a 28-year-old whom some teams and pundits still think could develop into a solid player, or to shove him into a platoon role that hides both his poor framing and his scuffles against right-handers. Montero is a welcome source of stability at an important position. He allows the team to take smart risks at other spots and modulate itself as needed.

On the Arizona side, there’s a whole lot of nothing. Jeferson Mejia is a nice pitching prospect, in terms of upside, but he’s a 20-year-old who has only reached the Rookie-level Arizona League. Zach Godley is a 24-year-old relief pitcher, and here’s a good test to use when trying to decide whether a 24-year-old reliever is a valuable commodity: Is he in the Major Leagues, dominating opposing hitters? If not, he’s not worth your time. Arizona wanted to clear Montero from their books, and to do so, they had to take hardly anything in return.

Montero is only a building block for the Cubs. He’s neither a cornerstone nor a capstone. He just fits into the structure they’re building. For both the Diamondbacks and the Cubs, this was a significant move, but mostly in the context of what comes next.