As mentioned above, its a combination of the Steam updating method as well as the file organisation of the game software.
Yes people hate how Witcher has an abysmal update functionality. What about other games though?
Nearly all other Steam-based games are much better with regards to updating. Updates are easily accessible and install/download automatically.
Why is Steam so popular? Its a game software manager, distributor and community. Steam allows for your games to be managed the same way, and be located in the same place. Updates can also be downloaded without say searching through development blogs. The e-distribution model is suitable for the current speeds and quotas of private consumers, and provides relatively easy access to content. Games can be downloaded at your own pace, and on any computer.
Binding games to accounts suits the electronic distribution model used by Steam. Account-based activation of games allows for the game to be played by the account-holder anywhere. It does however prevent the transfer of individual games between people and puts your access to games at the digression of Valve.
The popularity of Steam also allows for communities to develop among Steam users. As a system used by many game players, steam has both the player base and the infrastructure available to effectively support a gaming community.
I use Steam because it allows me to download games and chat and interact with my friends. It updates my games automatically and allows me to install them anywhere. I bought DCS on Steam.
Sure I might be positively raving about Steam. I've presented my points objectively and passively. I feel that being an active user of Steam I would know at least a bit more than those who dislike it and state they do not use it.