Tags can be used if you want to be able to send a sequence to a specific group of people that are in the same list.
You might have a client called a Medical Centre. In the Medical Centre, you will have Salons, Hairdressers, Dentists and various other services. They all will be on the same list, but with different calendars. Creating a tag for each of them in the form area of the booking app, will enable you to send emails to just a certain group of people.
For example: Your client (Dentists) might want you to send out an email to all their clients with their upcoming special. Using the tag “Dentists” that you have created, you can send just to those people on the list.
The benefit of using one list, is to make it easier for the Centre Management, in case they want to send an email to all the clients of the Centre.