She bought it because she likes it over a regular bicycle.

Just to add, there can also be commercial reasons for a tricycle. In my neighborhood growing up in Miami, there was an ice cream vendor and a knife grinder who went around on trikes. In the latter case, the chain was switched over to the grinding stone to provide power.

That being my wife's reason. She fell in love with recumbents in college, after trying one that a professor had, and bought a Hase Lepus Comfort recumbent tricycle a few years ago. She likes the stability, being close to the ground, sitting on a real seat instead of a saddle, and the cargo space for groceries or camping gear.