Questions about advanced use of oscilloscope
If you want to measure the rise or fall time of a digital signal, you use the variable knob for voltage to adjust the amplitude, so that it fits between the horizontal dotted lines (5 divisions).  Then you measure the time from 10% to 90% (the middle 4 divisions). Conveniently, there are even percent marks on the screen.

It's less common to use the horizontal variable adjust, maybe it could be used for measuring duty cycle.