Why?
Because it's badass, and you can still get copies for cover price on Amazon even though it's out of print. You can also get the PDF for $20 at RPGNow. (Disclaimer: That last one was an Affiliate link.)
Like the TV shows, the RPGs can be taken separately or complimentary with each other (in fact, my actual favorite combo is Angel combined with The Magic Box supplement for Buffy). These RPGs hold a very special place in my heart, in part because I love the shows to death, and in part because the larger Buffyverse line holds my first RPG writing credits (including a Buffyverse related article for Eden Studios Presents vol. 2 and an additional writing credit on Angel). Three times I have attempted campaigns (one a Buffy playtest campaign, one an IRC based online game and one an Angel game), and each time they just never "clicked", even though I was super enthused and everyone said they were having a great time. I even prepared a fourth campaign, which could be best described as The Dead Zone meets Friday the 13th The Series meets Beauty & The Beast, but we never even made it to our first session. I had grown quite adept at getting the Cinematic Unisystem to do the things I wanted it to do.