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.
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. 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.
Flexibility - Planes can be available at a few hours notice and can wait for you if you're running late. It's even possible to change flight plans in mid flight if necessary.
Quality of service - Private planes provide luxury furnishings, plenty of space, individualized attention, and your preferred food and drinks can be ordered ahead of time.
Privacy - You can hold meetings and make productive use of your time, without being over heard. Your overall travel will also be far less visible when you're on a private plane, so helping preserve the secrecy around important negotiations or deals.
Reduced stress - Without the lines, waiting, lost luggage, transfers, delay concerns or security issues of commercial flights, you'll be a lot more relaxed.