>>53480644For Incineroar specifically right?
In a nutshell, CP calculation weights the attack stat more heavily than defense or HP, as a result by sacrificing attack you can have a larger stat total for the same CP cap.
In this case for Ultra League, by dropping 5 attack, you can gain 5 defense and 6 HP.
The PvP battle format with how damage calculations are performed, favour making sure you have enough bulk to survive an extra turn, or shave off 1 more point of damage per fast attack received.