What people use it for
- Mixing Set a delay that lands on the grid instead of drifting against it.
- Sound design Turn a tempo into hertz for a tremolo or an LFO.
- Recording Know the sample count before you nudge a track by hand.
The arithmetic
A quarter note at 120 BPM lasts half a second, so the delay time is 500 milliseconds. The formula is 60,000 divided by the tempo, multiplied by the fraction of the bar you want. Dotted values are one and a half times the plain value, triplets are two thirds of it.
Turning that into hertz is the same number upside down: 1,000 milliseconds divided by the delay time. That is the rate to set on an LFO when you want it to breathe in time with the track. The sample count is the delay time times the session rate, which is what a plugin asks for when you type the number instead of turning a knob.