It doesn't reduce the amount of RAM you need at all. It does reduce the amount of VRAM/HBM you need, however, since having all parameters/experts in one pass loaded on your GPU substantially increases token processing and generation speed, even if you have to load different experts for the next pass.

Technically you don't even need to have enough RAM to load the entire model, as some inference engines allow you to offload some layers to disk. Though even with top of the line SSDs, this won't be ideal unless you can accept very low single-digit token generation rates.