Why are heater vents always next to windows and sliding glass doors?

Question

It seems like a huge waste of energy to put air vents next to windows and doors, but rarely do I see a house not set up that way. Is there a reason for that?

Answer 1

Because the cold comes through the glass doors and windows. Hold your hand next to them on a cold winter day. You will feel the cold.

Placing the vents next to the doors and windows heats the cold air before it spreads to the rest of the room.

Works in reverse in the summer when heat comes through the glass and a/c cools the air before it spreads through out the room.

Answer 2

Because this is the main source of cooler air entering the house. Having the vents there "tempers" the cooler air before you start feeling a "draft".

Answer 3

Because as the doors open the cold air pushes warm air in x*x