At the time most audio cassette interfaces for large computers used large reel to reel tape and smaller hobby computers used dedicated computer cassette players. Woz wanted to use a relatively cheap off the shelf player. He also didn’t want to have all that analog circuit that the computers like the Altair required for their cassette interfaces.
Woz would take a new approach to this cassette interface, something that is carried through to all his computer projects afterwards, he would use a combination of software and digital hardware to simulate as close as possible analog signal processing. This allowed his Cassette interface to be cheap and relatively un-affected by most consumer cassette player discrepancies in the analog signal.
Where most "hobby" computers at the time had 30 or more components for their "analog" to "digital" cassette interfaces, the "Woz" purely digital cassette interface had 6 chips.
This kept costs down and increased reliability. Apple was able to provide a mass storage device to the public for $75 dollars. A modification of this circuit was even included as part of the Apple-II.