If you are a mechanic with an auto-repair shop you spend a lot of time in your garage...
Fixing mechanical issues and offering maintenance services are only a few of an engineer's daily activities. Given your time is valuable, you should really focus on your profession rather than invoicing, billing and other time-consuming activities.
Some of the problems mechanics face
Poor Cash flow
Given the fact that you need to buy and install auto parts first and get paid by your clients vehicles, you have to be aware of your revenues/expenses balance at all times.
Mechanics usually work on several vehicles at the same time. Each needs to be delivered on a certain date; delivering a vehicle ready on time, is part of your professionalism.
Auto repairs are usually expensive and most of the clients can't afford to pay you upfront, so you need to be able to track due amounts easily and collect them fast.