In the base game, your legend knew one position. If you were a "Center Midfielder," you would never learn "Attacking Midfielder" unless the game's hidden XP system decided to teach you. With an editor, you can become a utility knife: CF, SS, AMF, CMF, and even a makeshift CB all in one save file.
Nothing is worse than playing 40 matches, scoring 25 goals, and then watching your digital avatar fracture his tibia in a meaningless cup match because the game rolled a hidden "injury resistance" dice. Editors allow you to set injury resistance to a permanent "A" (Very High). Pes 2009 Become A Legend Editor