Its not necessary to buy things from, you need it to run the games though. Its a very good service if you ask me, if not the best.
I hated steam when it first came out too, but it remembers what games are available to you lets you download and install them without disc, and just bans blatent cheaters.
Steam is great, you dont have to use it, but as a steam user for a considerable amount of years, I'd recommend it.