Cooper, mast rake hanges a few things and as a result a full explaination is complicated. But the main reason it helps is to do with lift distribution.

with the mast upright the centre boards are providing most of the hydrodynamic lift that stops your boat going sideways. In order to develop this lift your boat must go sideways a little (leeway) so that the angle of attack of the symetrical foils is sufficient to generate lift. If you rake your mast back then the cntre of effort moves aft so that now the rudders are also being asked to supply lift.

by splitting the load between the boards and rudders you have reduced the amount of lift required from the centre boards and therefor the amount of leeway you'll need to generate said lift. As a result you'll point higher.

As JW says the limit is defined by when you go vlock to block on your main, but before that happens you may find you've developed to much weather helm, making your boat hard to sail.