I wasn't aware of this rule until the recent fight over mentored youth hunting in PA.
A lot of guys kept their kid's license with theirs. So you don't lose, misplace, forget them. The mentored youth can only hunt with their mentors so they would never need to have their tags personally. The parent usually has a pack and keeps everything in the pack.
The big complaint in PA is that guys were using their kid's tag to shoot a 2nd buck. A weak argument based on paranoia and grumpy old men who don't want to compete with kids in their deer woods.