The cut outs helps on the early planing on bigger boards and also foil boards.
The board feel more 'free on the water' with the cut outs. The smaller the boards the less you need them.
Also with the cut outs you have better ability to 'pump the board up with your movements' , as with the less surface / cutouts (but still having the width), you can slightly change the 'trim angle' when you pump / accelerate more efficient and you don't have the whole surface (slightly draggy) to deal with
The cut outs are ate trade off from maximum width for leverage against a higher drag from total width vs narrower tail when with less drag to get on the foil
I thought the cutouts were to reduce wetted area at high speed, if anything they increase drag at low speeds. For a foil board which is only in the water at low speeds, I really don't see why they bother.
I don't like cutouts. They hurt early planing in my opinion.
I did some googling into tail cutouts and found one top race board with vent tubes drilled from the cutout inside corner to the deck. They were venting the suction/drag cutouts create when trying to plane.
Fanatic don't think they're necessary and don't have them on their StingRay foil boards. Easier to make without. Cut outs on foilers could be the influence of the marketing man?