I didn't watch them all.

Anyway, it's the same as with all other media. Anime isn't any different. My economy doesn't allow me to buy every anime I see just because i saw one interesting 2 minute clip. That would be like buying a house after seeing an image of just one of the rooms :). Of course, a house is a much bigger investment than a DVD box set, but the point remains. I've bought lots DVDs even though I've already seen them already, maybe even have them on my PC.