If you anchor the top of the sail onto the mast, then pull (say) 400lbs down on the foot of the sail, then you are applying (approximately) 400lbs force acting downwards at the top of the mast.
If, instead, you route the halyard over a block at the top of the mast and bring it back down to deck level (as per a conventional halyard) then in effect the masthead sheave introduces a 2:1 purchase. Whatever force you apply on the tail of the halyard is approximately doubled in terms of the downward pull on the top of the mast.
No, at the end of the day you still want your 400 lbs tension on your luff to get the same trim. It is correct that there is a 2:1 purchase but it only means you have to pull the halyard half as hard to get that same rig tension.