Steam keys

I only use steam occasionally and I've been wondering - what's the big deal with Steam keys? Why do gamers sometimes prefer or even demand to run their games through Steam when they purchase the game from another service?

For me personally its quite simple.
new PCs don't necessarily have optical drives (atleast not the ones i build for myself) and having all your games in one place, available for download on any machine you want whenever you want is really convenient (if you got proper broadband), I only buy games that are either reasonably small and DRM free(so i can store them on my own server) or on Steam(unless they got extra DRM from the publisher that adds further restrictions). Other digital distribution channels might be just as good or even better but i allready have a huge set of games on steam and don't really want to bother with yet another service.

I think that those gamers have accepted Steam as their trusted DRM provider. Going through another provider adds a barrier and is also a trust issue.

Do people still like Achievements? Might people prefer the Steam version 'cause they wanna get Steam cheevos?

I like stuff on steam when possible. It is my favourite and most trusted provider and portal. Toss the key in my account, and forget about it. I own several games with other online providers that I've simply forgotten I own. Every now and then I'll get an email from them about some sale or special, and clue in that I actually bought a game from them.

If it's a small indie game, I'll just get it from the developer rather than use a steam key (if available), but for larger games it's handy to just let steam do it's thing.

Most of the time if I'm downloading a large file from some random website, I get speeds of about 100KiB/s. Most of the time with steam I get 1.5MiB/s, because their CDN is awesome, so I'm actually connecting to a server in my city, instead of one 10,000 miles away.

I love having everything in the one place and being able to redownload it if I uninstall it.
I also like the cloud saves and community integration as well as steam workshop.