In some case, it could be useful to have an auto-increment number in order to easily identify and find a record into a big amount of data.
To create a auto-increment field, you can use different ways like using a plugin or even a real-time workflow but this means that you will have to create one workflow per field you want to be auto-incremented.
But with the new dynamics 365 interface, you have the possibility to easily create as many as auto-increment field you need without creating workflow or plugin.