To find the person they want to spend their life with. That's the reason anyone dates, really. Trial and error is the only way to find your soulmate. Though if you're taking about 10-12 y/o kids, it's usually because they want to act like "the big kids" and be cool.
I believe it's good practice as long as they are still being kids. Teaches them how to respect themselves & how to grow independently of their parents. You learn a lot about yourself & how to respect others in dating earlier in life than if you wait I believe.