Atlee makes one that is designed to be installed in a covered Supercub but it is very small. I had one in my cub early on and upgraded to the Cub Crafters baggage during rebuild.
1. Because it keeps the CG in the right spot.
2. You can load your heavy stuff in it that will kill you in a crash.
3. You can take it off and install it in 15 minutes.
4. Just easier to get to everything.