Well, for target bowling, the goal is to get as close to a target as possible.
In target bowling the aim is usually just to get the ball as close to a mark as possible. It would be pretty tough to roll accurately on gravel!
I love to bowl, but what is the games history, I wonder? I'm not sure about history, but it is a major throwing sport. Oh, really? Isn't there more than one type of bowling? Yes, there's pin bowling, which takes place on a wood or synthetic surface, and there's target bowling which can be done on grass or gravel. Oh, I couldn't imagine playing on gravel, it would be so hard to aim!
In target variations, the aim is usually to get the ball as close to a mark as possible.