They wanted him the season prior for the same reason they wanted him last year: he was their target in 2016 when they were getting Carr to accept his pay cut. Then they signed him this year when they didn't retain Carr again, as relatively affordable insurance for what they were going to do in the draft, where they ended up taking Awuzie, Lewis, Woods, and White.
As far as his play goes, I know you hated the guy from the outset, and I'm not going out on a limb for his play because he wasn't an adequate replacement for Carr from what I saw, either, but the guy played in a game and a half when it was said and done. He's been concussed for three weeks now, which is a long time to be out with a head injury. In the interim, Lewis has played really well, so it's pretty obvious why the move is getting made here over the bye week, and let's not pretend it's because the guy has been getting lit up here. He's not as good as the young players they've brought in, and he's had a serious injury in the mean time, so they're comfortable letting him go.
They've been signing 'garbage heap players' for years now. That's how they round out their roster while they find young guys to develop. They've done it since the 2011 season when they revamped the OL, and fans have complained about it ever since. They did the same thing with the DL brining in guys like Selvie and Mayowa. It is what it is. That's how they go about putting together their rosters. You don't have to like it, but don't pretend it's anything other than what it is.