I like epilogues in a couple of situations:

1. If I loved a fic A LOT, for me it's like a little extra prize. A little, 'they're still OK in the future!'

2 If a story ended on an ambivalent note. You know how much of a sucker I am for happy endings and sometimes I just like to be reassured things don't go to hell when I'm not looking.

So basically, my reasons are selfish and blindingly romantic.