The thing that makes Steam an authority in the gaming world, its tied in with Valve software. Valve = one of worlds leading game software developers.
^^ This
Valve has earned my trust in pretty much everything they do now. They make games for consoles, but have stated that they are going to make games for PC first. Instead of how a lot of developers have been purchased and make games for consoles then port to PC (always worse). Easier to downgrade controls, effects, textures, than the other way around.
Their steam app is fairly lightweight and is pretty stable nowadays. Many retail applications are often bloated with additional unnecessary files/applications. There are still a few features that people would like implemented, but valve works hard at trying to make sure they don't go overboard which leads to bloated software.
Valve continues to make free updates all the time to their games and software. The first DLC they made for L4D2 they wanted to be free to everyone, even for those on Xbox 360. Microsoft wanted to charge for the DLC, but valve fought with them over that stating they wanted it delivered for free. I think Microsoft charges for it, but I did read there is some DLC for L4D and TF2 that valve got them to deliver for free. Valve doesn't shaft their customers!
Valve continues to have great sales on games all the time. Hell L4D2 was going for $5 at one point I believe. In which you received all their updates and DLC for free. Summer sales and Winter sales becomes a time where you can buy games for 75%+ off. I know people complain about certain issues with being able to transfer games or how all the games are tied to an account, but obviously they should get paid somehow.
VS Apple... well apple is in it for the money. Look at their macbooks. You get the exact same internals in a macbook as another laptop, but charged $500-1000 more. If apple can get another dollar from you, they will.