What is the reason for the violinist's use of rosin on the hairs of the bow prior to playing the violin?

With the violin, viola or any fretted string instrument using a bow, it is only the bow that can make the strings produce sound. Accordingly, there needs to be enough friction between the string and the bow hair. In order to achieve this, it is done by increasing the friction between the bow and the string after the bow is rubbed properly with rosin. The rosin creates the necessary friction resistance and the bow can then make sounds come from the strings.

Musicians who are considered experts or talented, when playing string instruments such as the violin, know why and how to apply rosin in the correct manner. The end goal is to have just the right amount of rosin to create warm, rich tones, with just the right amount of friction that enables smooth movement of the bow over the strings. Too much or too little rosin can result with mishandling of the rosin in ways that are detrimental to the bow and/or the musical instrument itself.