7.5V can be useful if the voltage supply VS is less than 18V and your application need a regulated voltage for the gate drivers.
In this case the linear regulator cannot provide 15V (because its input is too low)
Power MOSFET are driven using the VCC, so higher is the voltage lower the Rds,on.
Switching loss due to the gate charge are higher with 15V (however we are talking about few mA).
My suggestion is to use VCCVAL = 15V if the Vs in your system is greater than 18V.
