Linked from
The 14 pages that link to Phase-locked loop, each with the reason it gives.
Atomic clockRelated: Feedback keeps the clock oscillator aligned with the atomic reference.
Phase (waves)Related: It uses phase error to synchronize oscillators and recover timing.
Clock signalRelated: Phase-locked loops generate and align clocks at frequencies suited to a system.
SynchronizationRelated: It synchronizes an oscillator by continuously correcting its timing error.
Crystal oscillatorRelated: A crystal often supplies the reference that stabilizes a PLL’s output.
Phase modulationRelated: A phase-locked loop can track a modulated carrier’s changing phase.
Clock rateRelated: Processors commonly use it to generate high-frequency internal clocks.