I love the floppy disk. Everything about it is just neat. The engineering is neat.
Drive motors weren't perfect and could spin a little too fast or a little too slow and could even vary speed over one revolution (wobble).
Now consider you could have data on a single disk that was written in multiple drives with different characteristics.
The data on the disk, although digital, was encoded via a very analog magnetic represention (flux reversals).
To read the data off a floppy disk the drive controller had to synchronize a data window clock with the rate the data was coming off the disk. To accomplish this, clock bits were encoded in the signal. In the PC world, this is called MFM encoding.