It's building electronic components vertically instead of laid out flat (think of a book stood on an edge rather than laid down, kind of). That has two advantages - they take up less room and they don't leak so much electricity to the silicon that they're sitting on.
The process causes the gate to be exposed to a larger surface area of the source, thus allowing it to have greater control. How? by going 3D. Why? Think of it as a heatsink. If your heatsink was just a flat surface, it would suck. But because it has fins (or pins, or whatever 3d shapes that come out of it), it has greater exposure to ambient air.