Every guitar and player is unique!
A guitar setup is the single most important thing you can do for your guitar and one of the most important things you can do for your playing.
To most players a setup means adjusting the action and intonation, but these are in fact only two stages of a 6-10 step process.
A setup will include making necessary adjustments to all the things that can be adjusted on your guitar, eg. nut slot heights and profile, truss rod, fret level radius and dress, saddle/s radius and height, overall action, intonation and pickup heights.
In general I will set a guitar up to get the feel that I think is right for that particular guitar. I also make adjustments to suit an individual player’s needs, eg. a hard ‘busking’ strummer will require a different setup to a late-night cafe jazzer.
Everyone knows what it is like, picking up someone else’s guitar and going “wow this is nice to play” or “this is hard”. Often the guitar (old or new) one likes the tone of, is put aside for another that seems easier to play – a professional setup will give you ease of playing on the guitar you love.
Even a new guitar needs a setup…
Typically guitar makers send out their products set to an ‘average’ factory specification, and rarely do they invest in making fine adjustments that will bring the best out of the instrument. The guitar will usually be fairly playable, but that setting may or may not be optimal for the person buying the guitar.