You need a Pocket Trumpet if:
you have a physical challenge that makes you unable to hold a regular Trumpet;
you want to fit a Trumpet of some type into your luggage so you can practise while travelling; or
you think they’re cute and really really want one.
Honestly, I can’t think of more good reasons. I have one because I can’t get a regular trumpet into the hard saddle bags on my motorcycle. I still end up strapping the full size cases onto the rear seat if I have to sound good, but they’re handy if you just want to practise on the road.