Im not entirely sure what you are talking about, but Im guessing you have this machine that can position something within three axes (plural for axis).
Do do this, the machine would be using digital encoders
. On rare chances a serious jolt to the machine can cause the encoder to jump. Under normal situations that reset (homing) isnt required - just an automatic safety check.
It is of course required when turning off the machine, because it doesnt count encoder movement when it is off.