Convenience - There are over 5,000 airports in the US that private planes can use (vs 500 airports for larger commercial aircraft). This means you can often land closer to your true destination. In the summer of 2006 the ban on liquids caused all sorts of inconveniences for people flying commercially, but private flyers avoided all this. Private planes also mean that you can travel with your special belongings such as instruments, sports gear, product samples or bring your pet in the cabin.
Though the early Lockheed Jetstar had four, most production business jets have two jet engines, mostly rear-mounted podded engine. If mounted below their low wing, it wouldn't allow sufficient engine clearance without a too long landing gear. The HondaJet is the exception with its over the wing engine pods. Dassault Falcon still builds three-engine models derived from the Falcon 50, and the very light jet market has seen several single-engine design concepts and the introduction of the Cirrus Vision SF50 in 2016.