A good setup will optimize the playability of your guitar. If your guitar is already easy to play, a setup may not be necessary, but most guitars will see some benefit. In addition to making adjustments to the action, a setup usually includes a fresh set of strings. New strings always make your guitar sound its best. If you have an electric guitar, a setup may include adjusting the pickups which can have some effect on sensitivity and tone.
Basically, a setup is intended to make your guitar play easy without getting fret buzz. Some improvement in tone (less fret buzz, fresh strings) may be obtained as a side effect, but it's not the main goal of a setup. If you hope to improve the tone of your guitar, speak to the guitar tech about that. There may be other things to try which are beyond the scope of a basic setup.