Spreader rake will change how your mast reacts to pressures from wind and control lines. More rake will turn loads into mast bend faster than less rake will. Loads include side forces from wind, compression from trapezing weight (additional leverage), downhaul, and mainsheet tension.

I still treat spreader rake as a method to fine tune the mast reaction based on the weight of the sailors on board. Heavy sailors will want the mast to bend less quickly in response to additional pressure (higher wind) so they will want less spreader rake.

My impression of this is changing a little through conversation with some of the a-cat guys but I haven't completely digested what they're doing with tuning spreader rake.