For a digital recorder, it is necessary to have a clock that tells the recorder when it should capture a sample of the audio. For example, if you are recording with a sampling rate of 48,000 samples per second, the clock tells the device when it is time to capture a sample. If two recorders are working, their clocks may be slightly out of synch and the audio from one device will begin to diverge from the audio on the other device when they are subsequently played back with a single clock used for both sources.