Meaning of Steve Wozniak's epiphany on register addition/subtraction in 1975?
This was 1975. Microprocessors were not common knowledge. Wozniak had some experience with mainframe computers, but most likely hadn't seen a 4004 processor ever. 

Reading the instruction set of the 6502 processor showed him that this wasn't a useless toy but a real computer. The original price for a 6502 was 25 dollars plus 10 dollars for the documentation (which you were encouraged to copy), compared to a 6800 for $300 including added chips that were required. 

Wozniak's epiphany was realizing that you could take a $25 microprocessor and turn it into a real computer. Apple Inc. is now valued a bit over 700 billion dollars, a direct consequence of Wozniak's realisation, so this was a kind of important event. 

Not that it has much to do with computer science, except that eventually it put computers into everyone's hands.