Linking games to Steam provides the library aspect and social features I believe, but I wouldn't argue it has all of the upsides of a Steam purchase. For example, I would buy Ubisoft releases way back when on Steam instead of Uplay because I could have it manage installing/uninstalling the games (even if I could've purchased it at a cheaper price via Uplay. Ghost Recon Wildlands comes to mind).
For library management, GOG 2 (Alpha) is the best solution I've personally seen (have the various storefronts installed, import the connector for GOG 2, all the games from that storefront are imported into GOG 2, can install/play/etc. on a whim).