Wiring these things was real slow, hard to follow, and nearly impossible to find errors when something was wrong. If something was wrong you literally had to follow every single wire on the board to try to search for the wrong one, and there were tens of thousands of these wires. To add a new program, designers had to go back to the patch boards and wire a new program. While it is true this happened, it is urban myth that the term came from this time. This was better, and allowed quicker changes, but humans would still make lots of errors trying to deal with patterns of 1's and 0's.
Mnemonics are just human descriptions for the bit pattern.
For example move, jump, branch, with some parameters. Assembly and machine code are very detailed. Each instruction does very little real work. Place food on the fork by shoveling or stabbing it with said fork. Open your mouth and bend your right arm and bring the pointed utensil towards your mouth .