Linked from
The 30 pages that link to Pendulum, each with the reason it gives.
OscillationBroader topic: A pendulum provides a familiar mechanical example of oscillation.
Mechanical energyBroader topic: Its changing height and speed illustrate mechanical energy exchange.
Phase portraitBroader topic: Its phase portrait distinguishes oscillation from complete rotation.
Hamiltonian systemBroader topic: Its idealized motion is a simple example of a Hamiltonian system.