Sorry missed the last part of your question.
Strikers and Defenders don't level (as you do) - hence the reason why you need to "upgrade" them.
Striker and Defenders 10% to attack or defense (respectively) when they are in your top squad. They also act as a normal top squad member by increasing your attack or defense. Like you remove normal members with other members (who have better stats) in your top squad (to ensure you have the strongest squad you can), you also upgrade your Strikers and Defenders.
hth
Last edited: Oct 12, 2010