I believe the player role options just correspond to preset slider positions. So for example, if you pick a player's role as Inside Forward with Attack duty then the sliders will be in one set of positions. If you change that player's role to Defensive Forward with a Defend duty, the sliders will move to a different set of positions. However, if you manually move the sliders back to the Inside Forward, Attack positions, then that player will play exactly the same as if his role had been set as Inside Forward, Attack.
The nice thing about the roles though, is that the sliders will adjust according to your team instructions whereas if you set them manually, they won't.
The nice thing about the roles though, is that the sliders will adjust according to your team instructions whereas if you set them manually, they won't.