The recurring bill process does not give you an opportunity to do certain things that are available during the "regular" billing process. For example, you cannot compose the text of the invoice during billing. Also, billing amounts are automatically spread proportionally across all WIP in the specified engagement, and this breakdown cannot be edited.
Recurring billing does not change the behavior of the invoice printing process. In other words, the detail region of the invoice will appear as usual — the descriptions are based on the invoice format (such as By Activity) and the WIP that was billed by the recurring bill. The recurring bill does not automatically create an invoice with a single, simple description unless you have specifically created a custom invoice format for that purpose.
Unless specifically set up to do so, a recurring bill does not eliminate the need to actually enter WIP before you can bill in a month. If there are occasions when no WIP is entered for a client in a given period, only the progress bill method would allow a recurring bill to be created. You will then need to perform some additional steps at some point in the future to close out the accumulated progress bills against any accumulated WIP (as the billing amounts do not directly relieve these transactions).
If the approval/posting process is enabled for billing, using a recurring bill does not eliminate the requirement to go to the Review tab and approve/post the invoice.