Time saving - this is one of the most significant reasons for using private aircraft. Depending on your program you can have an aircraft ready in just a few hours. You can arrive at the airport just minutes before your scheduled departure time, fly directly to your destination (without any layovers), make productive use of your time onboard, avoid overnight stays (saving hotel $ as well as time), avoid waiting in lines at the airport, land at over 5,000 airports in the US and so be closer to your destination. All of this can provide significant savings in both productive time and in dollars. Productivity - the time savings above provide significantly more productive time, both onboard and before and after your flight. You and your staff can make the most of the travel time to talk business or work with customers, suppliers or partners.
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.