Mapping is usually carried out for performance to improve the response from the engine or power output. The ECU (computer managing the engine) programmes how much fuel is put in, timing of the fuel, spark timing, airflow etc. etc. to keep your engine running efficiently. The programme managing this can be altered (re-mapped) to change any or all of these parameters in managing fuelling to the engine. Some claim improved economy but in my opinion this is minimal and may just be the computer reporting false better economy due to the changes and not real life fuel savings.
I would doubt you would ever see the returns to pay for the remap if that is your main reason, but I'm sure others with more knowledge than me will have a view.