You get to ride the downhill bits knowing that you've earned them.
You get to cover enough distance to see some fantastic views and wildlife.
You get to eat and drink lots without worrying about weight.
Depends if you mean XC riding or racing. I race occasionally because I enjoy competition, enjoy the challenge of riding fast and smooth and because I like the feeling of utter wastedness afterwards.
Having said that, I'm not very good.
If you ride with good XC racers (and I've ridden with the odd elite rider), you get to realise how much skill and technique they have in addition to the raw fitness.