With my psychology hat on:
Music can act to suppress outside (sound) influences while you are trying to concentrate, if it is music with which you are familiar - otherwise you tend to give it attention. Lyrics can be very distracting, especially if you are trying to work in a language which is not your native tongue, unless the lyrics are in a language with which you are not familiar, eg. I often listen to Ofra Haza while working because I can't understand Hebrew.

With my hard hat on:
This subject has been very heavily researched, so anyone asking about it in such vague terms is trying to re-invent the wheel or sell something. The general opinion is we don't want to buy it ...