I find it useful to be able to distinguish between IFComp Winner, Spring Thing Winner, etc. The comps all have different personalities, and their audiences value different sorts of games. Lumping them together wouldn’t be helpful if you’re trying to search IFDB for specifically IFComp winners, for example, which I have done before!
Maybe if IFDB had a better way to search games by such criteria, then you wouldn’t need the tags. But the tags are currently more convenient than anything else. Without the tags, you’d have to visit the competition pages individually, which gets cumbersome.