What is the purpose of VFR-on-top?
The VFR on top clearance is often given with a block of altitudes.  Such as this from the reference document "Maintain VFR-on-top at or between six thousand and one zero thousand.”  With this clearance the pilot can fly any altitude they want within this block of altitudes without having to contact ATC for further clearance.  This flexibility can be useful with the conditions are such that the clouds come and go as you fly along.  So if you are cruising along at 6500 and encounter a cloud and need to climb to 8500 to maintain clearance you just do it.  No need to contact ATC and wait for them to ensure they can clear you higher while the clouds keeps getting bigger and bigger in the windshield. 

Even if there are not cloud issues, the ability to change altitudes may be useful in seeking an altitude with more favorable winds.