Why promotion is forced
Why is promotion forced? Think about it...supposing you could not promote, now what would happen to the pawn?
The pawn would be stuck on it's square. Useless! Absolutely useless!
Promotion is much better!
Queen promotion is usually best. One can underpromote. Which means promote to something that is not a queen:
Usually rook or bishop promotion is useless, as you could get a queen:
If you are not going for a queen, what would you want?
Probably a knight. Knight can do what queens cannot:
This is why promotion was invented.