It flattens the entry to sail. It does allow you to carry it higher but it also moves the forward vector further aft (am I saying that right?) giving more healing moment with less forward speed. The trick is knowing when to pull the plug, just because you can carry it doesn't always mean you're going faster. As with anything sailing it just depends.

It's really easy to see. Go out on on a light air day sail as high you can with a full hoist then drop halyard the change really jumps right out at you.