Your doors have to a lot to do, particularly exterior doors.  Essentially, they need to prevent things you want to retain from escaping, and keep things that you don't want out of your home, all while allowing you to come and go as you please.  "Things" that might be as small as molecules of hot air, or as large as a burglar.  That’s a tall order for a simple rectangle on hinges!  

And since any door, particularly a front entry door, is a major design element of a home, it must be aesthetically pleasing as well as functional and efficient.

Read more about Energy-Efficient Doors...