The UPH proves you have a ticket that entitles you to park admission and has been used earlier in the day. The hand stamp proves that it was you who was in the park earlier that day, not someone else.
As mentioned above, it's always the wise thing to get a hand stamp when leaving a park. Gate CMs can and do refuse people readmission if missing a hand stamp although it admittedly doesn't happen very often. But why chance it?