Three reasons (for me that is):
1. Firmware region cracks are widely available. Even though I live in Japan (region 2) I can watch Region 1 DVD's from the US. Not many set-top DVD players can do that (unless it's some dodgy Chinese model).
2. Different TV-systems are not a problem. I am originally from Hungary, but currently live in Japan. Hungary (like most of Europe) has PAL, while Japan (and the US) are use NTSC TV systems. With a regular DVD-player you cannot play discs that are encoded in a different TV system. However, PC monitors do not care about TV systems, plus you can pop in a PAL DVD in a PC, use your video out to hook it up to an NTSC TV, and let the computer take care of the frame rate and resolution differences. If it weren't for computer DVD-drives, I would not be able to play DVD's from Hungary.
3. Sooner or later the disc media of choice will be DVD's. Some games are already available on DVD. One of the PC magazines I get frequently comes with a DVD. While DVD data discs are not that common just yet, it's good to have access to them.