What are them 'Enable' +&- really for? since in most cases they're not needed connected.
Depends on the driver make and model. Some might not even have an enable/disable input.
Some need it enabled with current thru the opto.
Others disable when current through the opto.
The + goes to +5v through a resistor, typically 330 - 680 ohms.
You can use 12v and a 1K resistor or 2K2 1/2w with a 24v supply.
The - side is switched to 0v by a BOB output or a switch, or a part of the machine safety circuit.
When disabled the stepper is free to turn. (no motor current)
It can be used to freewheel an axis.
Never disconnect stepper wires to freewheel an axis. That will probably destroy the driver.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.