I'd like to know why people use red arrows on strikers (ST) and wide attacking mildfielders (AML and AMR)?
I don't understand why should I put one of those on attacker, to force him to join attack? If he is in attack, why order him to join attack LOL?

Instead I'd suggest blue arrows on strikers for 3ST formations. If someone doesn't understand why, then I'll write Wayne Rooney, attacker who has great work rate.
So If you use such offensive 3ST formation, then you need 1-2 strikers to help in defensive (sth what Rooney does).
And reverse:
If you are playing some 5 defenders formation, then you could use red arrows on some of them, but be careful:
-When opponent uses AMR and AMR against your 5DEF formation, then do not use red arrows on fullbacks!
-When opponent has 3 offensive players in front of your defence (eg 3ST, 2AM+ST etc.), then do not use red arrows on DC's

Remember that there is no universal setup which is good, so that's why we can adjust our formation and tactics. That should improve our stats in matches.