You get a lot more variability in flow rates with that type of system.  With a fixed displacement pump (like on your 750), you are always pumping "X" GPM.  If you want to spray less than that, you have to bypass whatever fluid you don't want out of the booms.  Too much bypass and you can run into problems (foaming for one).  If you want to spray more than "X" GPM, well, you can't.

With a variable pump (old Rogators were often hydraulic servo control, new are hydraulic PWM control), you can run (basically) as low of flow rate that you want as long as it is high enough to create enough back pressure to get consistent flow (small tips or a low rate kit that pushes all flow through a 1" hose prior to going to the booms) and as high as they system is designed to flow.

This gives you the ability to run many different rates, at many different speeds, at many different pressures.  In my experience (mostly with self propelled, not a whole lot with pull type) the self propelled sprayers with variable pumps hold the rate a lot better as you change speeds.