I go on frequent bike rides in order to get more fit and more skilled at riding. The reason I want to increase my fitness and skill is to permit frequent rides. Biking is both its own requirement and reward for me.
Also, because I enjoy being outside, it's a good way for my wife and me to spend time together, it serves as a social utility with friends as well, there could conceivably come a day when I'll have to turn to bikes for transportation, and a bunch of other reasons. But mostly because I just like to ride. I didn't know I was supposed to have an end goal.