It's very easy for the outer edges of a quilt to stretch, and if you just sew a strip on and cut off the excess, you can end up with wavy borders. These can be very difficult to quilt nicely without pleats and puckers. If you'd like a handout on how to properly add borders, you can find one here under "How to Apply Borders"
http://www.andicraftsquilting.com/learn.htm