Not sure why we collect keepers, either.
Since about 2007-08, we seem to have usually had three in the first team, often because of "overlaps", when a new keeper is brought in clearly to succeed someone else - this happened when Foster came back from loan and we ended up with Edwin, Ben and Tomasz Kuszczak. Eventually, Kuszczak was fed up being third (occasionally fourth, behind Ben Amos) and left.
At other times, we've just worked with two - after Kuszczak left, effectively Amos and Sam Johnstone doubled up as first team backups and second team main keepers. Almost everyone I know who saw Amos play thought he was awful.
The reality is, we've rarely needed a third keeper, so the risk is low if you don't have one, as there's usually a second team keeper who's good enough to step up for the odd game. In theory, of course, you could be caught out if both the main keepers became unavailable for an important match.
From what I've seen of Grant, he's no better than the second team keepers who could step up, but if the view is he performs a useful role (coaching, etc) and he doesn't cost that much, it probably doesn't matter in the big scheme. I was a bit baffled why we brought him in at the time, though.
Beyond the "main" three, I don't know why we need six second team and four Under-18 keepers (according to the current player listing). I can see why nowadays you may want to have the youth teams separate as development squads, but inevitably some of them are out most of the time on loan until we release them, as we haven't brought any through to the first team in recent times. It's great to have a promising player like Henderson on the books, but what do we do with him, other than some people suggesting he provides De Gea with a reason to up his game? He will probably be happy to be on "loan" again at Sheffield for a season when football restarts, but beyond that he will probably move on anyway unless De Gea leaves.