It has since gone into a bit of a decline, which explains my fellow countryman's lack of knowledge.
The main purpose of mouth music was the conveyance of music in the absence of scripted music. In some instances the mouth music was incorporated into the song. It's a bit like having an instrumental in the middle of a song - except it's played by the voice instead of an instrument. Pipers still use mouth music to orally explain fingering, runs, throws etc in a particular piece. Mouth music may have become more popular with the banning of the pipes (never happened) after the 45 but it was practiced well before CE Stuart traipsed about the heilins. The instrument that really took hold with the banning of the pipes (never happened) was the fiddle with great composers such as Daniel Dow producing their work in the late 18th century. The decline of the pipes had more to do with the breaking up of the clan system and the lack of sponsorship by the Chiefs combined with economic migration to the cities and colonies, not very romantic I know.
It may exist but the reasons cited above for it cited have long been disproved. The Pipes (nor Scottish folk dancing) were never banned after Culloden and for Scottish above read Highland. For answers on the banning of Kilts and Pipes see separate answers.