To build up our personal libraries on IGDB, we wouldn't have to manually search for and add each game. We could simply connect our IGDB accounts to our accounts on all of those services, and IGDB would automatically populate our libraries with the games that exist in our libraries on each of those retailers. Steam, for example, has a 'sign in with Steam' function that IGDB could implement, and we could use it to connect our IGDB accounts to our Steam accounts. Other game retailers have similar functions; that's how GOG Galaxy 2.0's integrations work.
This way, in IGDB I'd be able to create my universal game library more quickly than manually searching for and adding each game from each retailer, one after the other.
Furthermore, our universal libraries would include games that we have via subscription. For example, games from the Microsoft Store via Xbox Game Pass and games from Amazon via Amazon Prime. When we connect our IGDB accounts to our Microsoft accounts to sync our Microsoft Store games to IGDB, it would include games that we have via Xbox Game Pass. When we connect our IGDB accounts to our Amazon/Twitch accounts, games that we've claimed from Prime Gaming would show up in our IGDB libraries.