No matter what type of moderation you're doing, there should be a reason behind your action and why you took that action.
Without reasoning it can be hard for doing ban appeals or looking at the past of the user's infraction records. So it's always a good idea to have a clear reasoning and make sure it is against the Discord Server's Rules and they are violating a valid rule of the server. You should also have sufficient proof recorded in logs or proof posted in your Mod Log.
Situations where good reasoning can help:
You're looking through a ban appeal and you don't know what they did wrong.
Looking at their previous infractions to give the appropriate action on a new infraction.
Making sure there is a reason for the action and the staff is not just abusing.
Where you might need good reasoning:
No matter what moderation action you take you should have significant reasons behind why you took that action. This can help for the Person who was punished to understand what they did wrong leading to that moderation action.