The term tune-up, to me, implies that we go through machine. We clean it inside and out. We remove lint and old oil and grease. Old oil and grease get stiff, sticky, then hard and act as an adhesive. Often the reason zig zags and reverse linkages don't work is because of old oil and grease.
We free up all linkages and check belt and bobbin winder for wear.
We machine away any needle damage. Every time you break a needle, you damage your machine.
We replace damaged parts. Plastic and rubber parts only last so long. When they get brittle they break easily.
We check stitch quality and set timings and clearances as well as balance tensions to get a very consistent, quality stitch.