Honestly though the biggest benefit they provide is the organisation. Having multiple editions of albums grouped together and comprehensive organised discographies makes it very useful to find exactly what you're looking for. It's like if Discogs had a download button.
From what I've heard Oink wasn't organised like that, you had to search for everything (I wasn't on Oink so can't say for sure) but What.cd was. and Redacted (along with Apollo which is the other big one that popped up after What's demise) continues that structure.