I have personally repurchased several games that I already owned physical copies of during Steam sales, just because I like the convenience of having digital copies and a shared library between my various computers. I move around a lot as part of my job and you wouldn't believe how nice it is to be able to pack up all my old PC games and give them to my brother because I can access them at any time on my laptop while posted in Afghanistan.
Also, I've been a PC gamer for quite some time, as far back as playing lemonade stand on my apple 2c, or Sim City on my 486 after that. I have a certain attachment to PCs as a gaming platform, and a lot of genres that I have a deep love for, such as strategy, only seem to be widely available or done correctly on PC. Since the last generation of consoles, the area I live in saw a drastic reduction of PC games in their game shops. EBGames, Future Shop and The Source all cut their stock drastically, down to maybe a shelf of exactly what you could get on consoles, with the addition of The Sims and Blizzard's games. I simply could not find anywhere to buy my games for my system of choice; I even had to turn to Ebay on a couple occasions. Then I activated Civilization V on Steam, and was just blown away by the amount of titles I had been looking for, and at decent prices; especially once the Christmas sales hit shortly after that.
Since Steam I really turned around my stance on digital distribution as I saw how well it can be handled. I've bought games off of GOG.com, quite a few for my dad, which can't be found anywhere in physical form. I even installed Origin to check it out, give it a chance and buy The Old Republic. Not a big fan of Origin, just feels like a clunky interface and there isn't a lot to offer that I couldn't just get on a console, so I just set my TOR to open through Steam and bypassed it.
Sure there are a few beefs with Steam; like their offline mode could use some work, as I found out when I tried taking my laptop to game with while on a training exercise away from wifi. But I find overall the program to unobtrusive, you're not constantly bombarded by advertising if you just want to pop into your library and launch a game, the chat feature is nicely implemented, and of course the price is right.