On a few slower/lower memory machines, users reported "sluggish" behaviour when using other programs. These settings were invented to provide those users with a way to limit the ram available to boinc projects, and not force them to stop contributing all together. I've got mine set to 100 each and have yet to see any sluggishness, but then I have the luxury of only using my fastest machine/s for everyday things and leave the rest to just do boinc.

You don't need to try and match useage to the settings, but can if you wan't. The higher the setting, the less frequently swap space will be utilized. Swap space if very slow compared to L1, L2 or memory access.