On x86, some instructions are longer than others. Incrementing is a single byte. Setting the value 1 if you don’t assume anything is going to be 2 bytes (al, ah, bl, etc.) or 4 bytes for (ax, bx, etc.)

This online x86/x64 assembler is great: https://defuse.ca/online-x86-assembler.htm

Saves one byte, I assume. Register targets are typically encoded in the opcode while direct values follow the opcode byte.

Probably because (IIRC) mov is 2 bytes and inc is only 1 byte and they're optimizing for minimum size.