This would be ideal for support and development.
As a service provider, I'd like to be able to sell support packages
A package can be, for example, for 3 hours of hosting support.
A customer raises a ticket for support and if they have bought the package, we can start on it right away, otherwise, we send a link to buy a package.
An in-app timer could be useful. We start it once we confirm to the customer that we'll be working on it right away and we'll stop it when we're done, giving us an indication on how much time it took us to perform the task.
Once work on the ticket has been completed, we can remove an amount of time from the package, in 15 minutes increment per example.
The customer knows at all times how many hours/minutes are left.