Essentially you're spinning waiting for the event that you know must come. Your latency will be on the order of a few clock cycles. Whereas if you use interrupts you are always at least a state-save and state-restore underway before you can service the next event, and likely a lot more than that.

The difference is real time control of stepper motors with crazy speeds when microstepping at 300K steps / second or more interpolating across 5 axis vs moving them at a snails pace of maybe 10K steps / second (on really nice and otherwise idle hardware, miss a single step and your goose is cooked, now you have a cumulative error).