Almost all competitions have different restrictions/categories to participate in (swimming has backstroke, butterfly, freestyle, etc, etc)

Different categories of a competition challenge different things. There are very different engineering challenges related to building a boat out of a pumpkin. It doesn't make a better boat than standard metal/wooden boats, but it does cause people to think about boat building in a different way. It might just be a fun personal challenge, or it might be a fun way to think up classic boat solutions by looking at things from a completely new lens.

Worst case this wastes someone time and money (which a lot of standard engineering does too), best case someone has fun and/or learns something.