In what situation could the maximum ramp mass limit the allowed take off mass?
@vasin is correct. I'll put some actual numbers in a scenario.

A 747-481F has a max ramp weight of 913000lb, a MTOW of 910000. That 3000lb difference between max ramp and MTOW has been the same for 747s since the -100 airplanes.

The 3000lb is usually sufficient to cover the taxi fuel, but not always. Think having to taxi out at JFK when departures are heavy and the runway in use is the one farthest away from your ramp. Or, same thing at Honolulu and because of your weight you need the reef runway.

The dispatcher/load planner for the flight believes in following the rules, and he believes the taxi out will consume 4000lb. Thus, he has to use 909,000 as the max takeoff weight. Thus the max ramp mass has limited the MTOW

This kind of thing happens most on freighter flights because they're loaded as often as possible right up to their max zero fuel weight, that's where the money is. So let's say it's a freighter flight, JFK to Tokyo Narita. With the airplane near MZFW, you can't load enough fuel to get to Tokyo, but if you stretch a string on a globe between JFK and Narita, it goes almost right over Anchorage, and Anchorage is pretty much at the halfway point. Indeed, Anchorage has been used as the refueling stop for freighters for years and for pax aircraft as well, but especially for freighters.

Anyway, the load planner will put as much cargo aboard as possible to put the actual takeoff weight right up against the MTOW allowed.