Ectomorph mesomorph and endomorph tests are basically to see what type of body you have and what should be the proper diet for you according to it. This one is best done for the runners who have different types of body and hence it becomes easy for them to follow. Ectomorph test is for the runners who are naturally thin and don't have much body fats in them. They are basically long distance runners and can eat a lot of food to gain weight.
Endomorphs test gives the results of the people who are naturally strong and muscular but you can term them as overweight. Mesomorph is those who have both muscular as well as good cuttings in their body.