Automations are for curriculum and role based training These are assigned by role and location, or may be assigned to all users. Think of your orientation training.
Employees will always take it, and it's usually only taken once. This training is automatic. It's possible a majority of your training may use automations. This helps standardize who gets what training and when.
Assignments are a one time assignment or ad hoc training. A good example may be that your store failed a sanitization audit, so you assign your staff some sanitization training. You don't want any new employees to join ExpandShare and receive their standard sanitization training that may be a part of their role certification, and get the same training immediately afterwards due to their store's port performance.
This helps you better control the training your team receives And remember, if it's not part of the standard curriculum, it's most likely an assignment instead of an automation.