You mean VFR on Top. The main advantage is you get to pick your
altitude and go much lower against headwinds. Since there is no
separation provided by ATC there is a lot more route flexibility.

Lloyd I. Cripe wrote:
A pilot friend with a relatively new IFR rating asked me the following
question: If you are on an IFR flight plan, what would be the advantage of
flying IFR On Top? What is the purpose flying IFR On Top?

I would appreciate your response to these questions.