Depends on your token/packs avalaibility. I use to train just one cause with a 50 green pack i am usually able to give him a star more, then move on the next.
That is due to retarded game engine and probably retarded formation for your opponent. Or a good counter by you. But as example i can set you that during the first half with 60% possession, on the second half with same formations but you having lower condition the possession will slip onto the opponent. (i myself proved it going after my ghost formation accounts). Even a Q 18 team, after taking 4 goals in the first half,could draw 1-1 the second. Yes, if you are winning 5-0 it's np, but when the time comes you'll be fighting on par the condition will make the difference.
Another example is how many times i was having ball possession issues in the second half with the first being dominated by me, and subbed 2 half condition MCs for lower quality but 99% condition ones: my possession came back 5 minutes later, and i easily won. (from Q35 to quality 22/23.)