When you take the ITB off, from Gerdy's tubericle, you only have enough length to make only one pass.
When you take it based from Gerdy's tubericle, you can get enought ITB to make as many passes as necessary to cover the defect, and double or triple the thickness by going back and fourth between the insertional area on the femur and the tibia. This is considerably stronger. I only use this method.