If the sail is hooked at the top of the mast and you pull down with 100 lbs of force, you have 100 lbs of tension on the mainsail luff and 100 lbs of compression that the mast has to resist.

If you have a halyard that turns around a sheave at the top of the mast and is cleated at the bottom (i.e., the halyard cleat at the bottom of the mast is what keeps the sail from falling down) and you apply the same 100lbs to the tack of the mainsail, you have 100 lbs of tension on the mainsail luff and 100 lbs of tension on the halyard. You now have two 100lb tensions that the mast has to resist equaling 200 lbs of compression on the mast.