I assume you're referring to the POINT 1 antenna? It's main purpose is it has a compass which keeps the arrow always pointed in the direction of the front of the boat. For example, let's say you pull up to a brush pile and the wind is blowing your boat north but you are facing south... the built in HDS antenna would show your boat facing the direction your boat is heading not where it's pointing.
Real world application: you can waypoint that brush pile, when you get blown off course you can immediately find it again without having to search for it because the arrow is actually pointing in the right direction. No need to have to use a marker buoy to show all the pot lickers where your spot is.
It's also double the speed. Cost is about $129, I think.
Does this help?