Shooting hard means that the ball will travel faster and therefore be harder for a goalkeeper to reach. Use this option if the goalkeeper has closed down your angle or if your shooting stats are not as good.
Due to randomness the amount of pressure your player is under can impact the accuracy of hard shots in particular, this is why it is not always the right option.
Shooting hard is useful when you are unsure of goalkeeper momentum and positioning as there is less emphasis on aiming for the corners, placing this shot down the middle can still yield good results