Because a charter flight is not part of a scheduled service, the flight will depart when you want it to. You are also able to choose which cities you fly to and from. Charter Flights are particularly convenient when you need to travel to a city where scheduled airline service may require multiple connections or layovers before you reach your destination.
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.