The basic way to home is using one pin, D9, this means you can only "test" one axis at a time. This was caused by the boards that are supplied by Eleks Maker that tied all "homing" to D9 by hardware. Specifically the L6/L8 boards.
Grbl does not restrict homing to only one pin. In fact grbl assumes a different pin for each axis. This requires a separate wire set for each axis. The single pin method allows a single wire parallel wiring, which is simpler.
D9, D10 and D12(Z) (D11 in grbl 0.8 and lower) are reserved for homing/limits.
For CNC use you always raise "Z" axis first. This is reflected in grbl's default homing settings. It "tests" Z first then tests X and Y simultaneously. This is to prevent damaging the work, or the spindle, during a homing operation. My 3D printer homes X and Y then Z. Not a problem unless you have unfinished work on the table. Properly they should raise Z, a determined amount, before X and Y movement. CNC usually homes to the top of travel, 3D printers home to the bottom, the table. My Laser machines only home X and Y, each separately.
To "home" Z there are two grbl methods that could be used. Homing Z would be running the laser module to a predetermined point by switch placement. Then you would set, through gcode, your distance to the cut in the work piece. The other would be to use the "probe" function of grbl to determine the height of the work and then raise the Laser to the correct distance.
Limits "tests" cause an alarm and stop all movement of all axis, spindle and coolant. "Soft" limits take effect when the travel reaches a particular axis maximum allowed distance as defined by EEPROM settings. Hard limits act the same but are tripped by switches rather than by distance. Grbl cannot test the "area" by hitting the switches. It only responds to home switches and Soft or hard limits.
You do save time by homing simultaneously. This only matters if you are using homing a multitude of times during a single session. On larger machines you would probably want simultaneous homing. On an A5 size machine it really does not make that much difference.