Whdload machines seems to be ok with 2-4 MB of ram.
That's fine, but the extra ram comes in handy for preloading much larger games such as Indiana Jones FOA (11 disks). Or CD32 adventure games etc.
On top of that, with 64MB or more ram you can easily load and edit CD quality stereo audio tracks without having to use any form of swap file or virtual memory, as the whole track will load into ram.
I think there's also some AGA demos that require 64MB ram as well.
It's also nice having a large ram disk to extract archives into.