Input Capture and Encoders
PWM points the timer outward: the counter drives a pin. Input capture points it inward. The counter free-runs, an edge on a pin tells the hardware "now", and the value of CNT at that instant is copied into a capture register before software has had a chance to be late. That last clause is the entire value of the peripheral. A GPIO interrupt can also tell you an edge happened, but by the time your handler reads a counter it has been anywhere from 12 to several hundred cycles — jittering with whatever else the NVIC was doing — and the measurement carries that jitter. The capture unit's latch has no jitter at all.