When you're recording something to digital, the way that it happens is the program takes 'samples' at certain spaced times. Recording at 44.1 takes 44,100 samples a second, 96 takes 96,000, etc.
If you're using multiple digital devices, you want to make sure all of these samples in these different pieces of hardware are taken at the same time and align or there could be artifacts in the digital recording.
A clock syncs up all of those to one clock to make sure they're all together.