Cassette, being cheaper than a floppy disk drive, was a popular storage medium in the 8-bit era. Some computers such as the Sinclair Spectrum, just provided a port to plug in an existing cassette deck. However, you couldn't do that on a Commodore; instead, they sold a 'datasette' which was fundamentally a cassette deck like the ones designed for playing music, but connected to the computer with a special connector; you couldn't use an off-the-shelf deck.
Clearly the datasette provided some reliability benefits, through uniformity if nothing else; you would be guaranteed to be using exactly the same model that the ROM tape routines were designed for. (And since the PET was designed for business as well as home use, it made sense to pay the cost of a datasette if that would improve reliability.)
But was there anything else special about it? I'm guessing Commodore could just as easily have provided a plug compatible with any off-the-shelf cassette deck, and made the datasette optional if you wanted the uniformity benefit, that the special connector was just to increase revenue by making everyone buy a datasette. But maybe I'm wrong; maybe there was something special about the interface, that allowed some analog conversion step to be skipped or some such?