Can someone explain me about the balanced players

If you buy a scout player they are balanced in every skill
if you see mourinho's team the players are balanced in every skill

My question do we need to train the greyed out skills (or give them other positions against 50/40 trainingspoints)
each season we lose 20% of all skills

example: does a mc need strenght (greyed out) in my opnion i they have no strenght you lose the battle on midfield

does a gk need speed (greyed out) in my opinion if he comes out of the goal he needs speed

Can someone explain me this

Thanks in advance