from a legal perspective its simply to differentiate between the park and other areas of the mountain. if a normal skier buys a lift pass they enter into a contract whereby they pay a certain amount and agree to ride in control and the mountain operators agree to provide a safe environment and mark obstacles. those same rules apply less and less when you start putting in features that are deliberately difficult. big jumps and rails come to mind.
the amount you pay for the park pass doesn't mean anything. its just a way of making sure that you sign the extra waiver and enter into a contract stating that you understand the risks you're taking by engaging in that activity. it doesn't necessarily mean you can't sue but it does draw a distinction between the regular mountain and the park.
it also keeps kids and inexperienced people out of the park. they might get a sudden impulse to try something. making them go to the office and pay $15 for a pass gives them time to think about that. on a practical level that also stops kids from getting in the way and potentially getting hit (see skier jibs little girl video).