I carry with me most parts that maybe necessary to repair most malfunctions.
Adjustments aimed at increasing the efficiency of the mechanical part of your piano are called action regulation. Frequently, relatively inexpensive adjustments can make an enormous difference in the sensitivity of touch, allowing you to play much more expressively. These adjustments restore the correct tolerances that were set when your piano was new, but which gradually changed over the years with use.
Reshaping of the felt hammers to bring them back to normal tolerance is called voicing. It is accomplished by making careful changes to the shape and density of the hammers. A properly voiced instrument can sound remarkably more beautiful. Voicing is the most difficult skill for a tuner to learn, and it typically takes years to master. The cost is inexpensive if done on the same service call as the tuning. The result is often breath taking.
I highly recommend the appraisal of every instrument prior to purchase. Pianos vary widely in quality and condition for the same price. Many need costly repairs or maintenance that are not obvious to the untrained eye or ear. I also offer free advice over the phone that can save you time and money when you are looking for a piano to purchase.