If the game isn’t redeemed on steam then you can’t use steam overlay. Adding the game to steam as a non steam game might let you use it though. Other than overlay I don’t really know of any advantages or disadvantages. Well you also don’t have ot worry about VAC bans if you are modding and don’t use steam.
You can get the steam overlay on any game. Yes, you can still access the game directory folders for your games if you buy your games on steam. Really the only advantage of buying your games on steam is that all your games will be saved on steam so you won’t have to worry about losing games/product keys like you would a physical copy.
workshop however doesnt’t do mods as we do here for save modding ect just for thing like object mods for putting new armors and weapons and characters in a game (usually only scource games like garrys mod) some even make new games based on the original using different characters and new abilities ect.
All Steam is is a nice centralized place to download/organise everything. You also get that overlay and stuff.
I don’t buy my games directly from Steam, but I do buy steam keys from key websites (they tend to be a lot cheaper). You then just redeem them and download. It’s a nice system tbh and you can save a fair bit if you shop around for keys.
Then there’s Origin. Similar thing but more sucky.