Valanice: Phantasmagoria, which is probably the worst reason I could have had. Sure, it's getting me a master's degree, but I'm having much more fun with Descent and Gabriel Knight.
I heard on Sierra Gamers that GOG had Phant and a few other Sierra titles. I was happy, because my attempts to patch old Sierra titles to work on modern PCs had been...well, patchy. And GOG has more than won my loyalty (and when grad student here gets some time and money, I've got a list of GOG titles to purchase...)
I have also been very pleased at the amount of Sierra titles on GOG as my game shelf will attest.