Roughly how big is it? I feel like my sense of scale might be a bit off.

But some thoughts based on how big I think it is.

The bars look too far apart to be useful as a grill, also a little awkwardly low to the ground for that, not that campers are overall opposed to squatting or sitting on the ground if it comes to it, you could maybe put pots, pans, griddles, etc on it above a fire, but I don't see any evidence of the concrete being discolored from that.

The bars also look too close together for it to be a bike rack, and the little bars going across it seem like they'd be kind of inconvenient for that purpose.

You could maybe put a cooler in it, it wouldn't be particularly useful for protecting it from bears with the open sides, but could maybe keep a raccoon or something from getting the lid open, or if you bring a cable lock it might be effective at keeping bipedal scavengers from walking off with your cooler or stealing your beer.

Also seems a bit low to be a hitching post for horses or whatever.

As far as being a firewood rack, it seems a bit small for that, but I guess if you're not as big of a pyro as me and all the former boy scouts I know are then maybe its sufficient for that purpose.

The answer that makes the most sense to me is that it's for skis, or maybe trekking poles or something like that. Lets you store them upright so they don't get stepped on or buried in snow, leaves, etc. you said that the sites are closed in the winter, which puts a dent in that theory, but was that always the case? The concrete is a bit busted-up, so they've probably been there a while, maybe years ago they allowed winter camping and a lot of skiers used the sites. Also would explain why some sites have them and others don't, if they don't allow winter camping anymore, it doesn't make sense to replace the ski racks when they're damaged or to install them in newer sites.