Each table has the ability to require authentication. Normally this is just the user table, but it is possible to have authentication against multiple tables.
For example, sometimes you may have user accounts in more than 1 table. Such as, a legacy system that you need to still handle or maybe you have customers in one table and staff in another. There are a number of different reasons. Ideally, you have just one table for your user accounts - although this isn't a requirement.