The Gunners targeted Debuchy early in the summer as a replacement for the departed Bacary Sagna and the deal seemed agreed to for weeks, but was not finalized. Newcastle wanted to make sure they signed someone to replace Debuchy before they agreed to let him go. Not coincidentally, they announced that they had bought Daryl Janmaat minutes after the Debuchy deal was officially made.
Sagna joined Manchester City on a free transfer this summer, leaving a gaping hole at right back. Arsenal had to sign a new player, and while Debuchy is not as good as Sagna, he has proven to be a dependable Premier League player. And the Gunners didn't have to break the bank to sign him.
Arsenal's summer will be highlighted by the signing of Alexis Sanchez, but had they not filled their holes, it would have been for naught. Now they have done that, filling in right back nicely, and allowing them to get back to the business of strengthening their squad so they can make a run at the top of the league this season.