"As a replacement for red cards" is an obvious use. I did not list it because you don't have a choice -- unlike with an injured player whom you can choose to heal or replace.

"To change formations" is a good one! I bought an AMC who is only called upon once every 8-9 matches when I have to get away from my favorite 4-5-1V and 4-1-4-1 formations. Fortunately he also has ST or he would be very bored. By the way, wrong thread, but I have to say it. I had gone 5 competition matches without ST scoring. I put my 4* AMC/ST bench warmer in today and he scores in a 3-1 CL win. I had to go back six matches to a 5:0 first round cup blowout to see a ST score previously... and it's the same guy. I have a feeling he might get another start soon.