Lots of reasons
The primary reason for getting a rig was Mrs B's bad back and her inability to stay on the (two wheeled) bike for very long.
It has turned into a real experience for both of us.
We do it because it's fun, it's comfortable, we (meaning her) can pack more stuff...the list goes on.
Also one of the best things is giving rides to kids who otherwise wouldn't be able to get on a bike due to some type of disease like cancer or a birth defect.
Seeing the smiles on their faces...priceless.
An armed society is a polite society. Manners are good when one may have to back up his acts with his life. Robert A. Heinlein