A reduced test case means you run less code to process the test case, which means your breakpoints trigger less frequently (and the remaining breakpoint triggers are more likely to be relevant to the actual bug). It also means all your debugging steps are likely to run faster and produce less data to sort through. Your log files will be shorter and easier to read/grep, etc.

Imagine being handed a sheet of 10 equations and being told "1 of these equations is wrong." Now imagine that someone came in and erased 8 of the correct equations - they just saved you a bunch of time.