Steam is a service to purchase/download games, it comes with many additions that you may or may not want.
Many games you buy nowadays - even if it's in a case, will use Steam (or their own prepriatory service like EA's Origin).
If you wait long enough the game you want will be on sale dirt cheap, usual times are mid-summer, xmas, and most other holidays.
A lot of Steam games get patched and automatically update, but some don't. Also some games are moddable with Steam and others aren't.
I think Cliffs needs Steam, but is slightly moddable (check out the Flight Sim section). EE also needs Steam, and has some great free DLC. Don't know about modding that.
For me personally, I like it, others don't, but it is pretty much a necessity for the modern PC gamer.
__________________
|