...because that's what they are.
Granted, there's not a "store" but it's the same idea as "where can I get the cheapest Android phone".
If nothing else, it's someone trying to crowdsource their web search.
That said, a canonical "where can I find Android ROMs" question (Community Wiki) would be a good idea.1
1 I don't flash ROMs to my Android devices, so I wouldn't be a good person to do that.