What does Category in Billable Time do?

In the New Billable Time form, what does the Category field do, and how can I put it to good use? The only thing I can see is that whatever I type there appears on the list of all Billable Time entries, but it doesn’t seem to control anything else or appear on any reports.

Categories are informational only. They don’t show on reports or invoices, but do appear in the Billable Time listing. I find them occasionally useful to see what kind of work a time entry was for, as I sometimes do different things for the same customer. The handy thing is you don’t have to define them in advance.

Category will be removed as pre-defined field. It will be converted into custom field.

This field was added before custom fields were possible.

Thank you. I need to show a customer job number in my billable time invoices for my customers where I only provide labour services. My customer requires that my invoice has their job number for each hour/minute that I invoice. I currently use the category field to record the job number in my time records.