becos specific players suit specific formations and tactics, for example, if you are playing hexagon 4222 or 4321, you should ideally have ml/aml or mr/amr instead of amr/amc or aml/amc who cant pull back and help defending and zone marking opponent ml or mr if any. Aml/amc or amr/amc should be used as a subtitution or such formation with amr/amc and aml like 4231, 3232. Hope you get it clear.