When the Minibrute is not receiving clock from an external source, and you have the ARP going, if you hit new notes it will start those notes "instantly" as you hit the keys, so you will hear what sounds like it starting slightly out of time, either too early or too late depending on when you hit the keys, so you have to have very precise timing for it to sound good.
When clocked to an external source, ARP running, when you hit new keys, it won't "trigger" those new notes instantly as you press the keys, but rather will wait until the next tic/pulse to play the new note(s) so that it will be in perfect time with the tempo.
Personally I'm not sure WHY the first option would be preferred over the second. I think the MB ARP should behave the same even when only receiving its own internal clock.