Hello there, Brighton.

The mileage tracking in QuickBooks Online is solely for reimbursement purposes. Then, if you want to mark the items as billable, this is only available in the Plus and Advanced versions. The feature to track mileage for estimated tax calculations is available in QuickBooks Self-Employed only.

I will help you by passing this idea to our management team. We always appreciate customers who take their time to share us their ideas/preferences. This will help us improve QuickBooks Online's services and features. 

If you have other questions, please don't hesitate to get back to this thread.