As the person in the cockpit has huge numbers of Horse Power available, and only brakes or wheel chocks to stop forward movement, applying that power six feet and more above said restraint, the airframe would nose over pretty quickly. So you need to oppose that, whether by mechanical or, more practically, human avoir du pois, or the tail will rise, nose goes down, end of propellor, and probably engine.
So, on a power check, where you don't want the aircraft to move, it is held on the brakes/chocks, with weight applied on the rear fuselage. No problem. Engine develops full power, no noted misfires etc.. all good and OK to fly. Reduce power, remove restraints and fly, or park up for later flight.
However, the same applies when using the engine to move the aircraft over (eg) boggy ground. Loads of power to get it to get it's wheels to move would also cause a nose over. Add weight. Move to drier ground. You are now ready to fly.
Best to check that the weight has been removed first!