It might be relevant to create a group if you wish to have a special set of people identified with a ‘flair’ on their avatar. That’s up to you.
Some groups are special, like moderators or admins, and that determines who can do certain actions.
But this also includes users, who are automatically added to the higher trust level groups if they engage in the forum in the positive way once they meet pre-determined criteria. You don’t need to do anything for this to happen. This gives the users access to richer functionality (e.g. ability to change title of another User’s Topic).
My suggestion would be to stick to the out of the box groups first. You will know when you need your own bespoke groups.
If you don’t think you need a new group, then you probably don’t.
As soon as you hit a situation where you want only a subset of users to see some information, you will definitely need a group. You might need to create a group if an automatically generated, default group is not suitable.