Optimising use of the A register
Writing code on an 8-bit microcomputer requires a skill that has been lost in the modern programming era. These days, developers are used to having Gigabytes of memory and processors that run at multiple Gigahertz.
In the microcomputer era we had far, far less resources. Processor clocks ran at 2 or 4 MHz, one thousandth of the clock speed of modern processors. If we were lucky we had 32K, 48K or 64K of memory to play with and that was it.
Because of this, we had to learn pretty quickly how to optimise our code to fit into memory. If we were lucky we could use a floppy disk to page in parts of the program as needed, but even then when a Cassette tape was the primary medium for a platform that wasn't even possible.
Optimisations at the machine code level would be a balance of reducing the size of code and having code run as fast as possible.
Sometimes you might sacrifice some memory for speed if the routine is important, for example it's doing a transform for some graphics in real time, but most of the time it's to reduce the memory used.
Optimising comparing numbers
Optimising bit shifting