Hi @guisalvi,
Welcome to the Auth0 Community!
The number of users in a B2C Essentials plan starts at 500, and can be configured for more, whereas the Free plan has a total maximum limit of 7500 active users.
As to the why ; you may only need a few users, <500, but want the features offered in a paid plan (dedicated support, custom domain, etc.).
Hope this helps!