It is all anecdotal evidence, but I am aware of some parents who "forced" their baby (and then little kid) to listen to classical music (specifically Mozart), because the kid would become a genius. Apparently, after some googling, it is called the "Mozart effect". Here are press articles about it:
[1] https://www.scientificamerican.com/article/fact-or-fiction-b...
[2] https://www.bbc.com/future/article/20130107-can-mozart-boost...
Could it be a motivation for teaching music to little kids? That is what is suggested in the conclusions of the articles linked above, and which I quote below. Some parents might be receptive to these conclusions.
From the Scientific American article [1]:
> Rather than passively listening to music, Rauscher advocates putting an instrument into the hands of a youngster to raise intelligence. She cites a 1997 University of California, Los Angeles, study that found, among 25,000 students, those who had spent time involved in a musical pursuit tested higher on SATs and reading proficiency exams than those with no instruction in music.
From the BBC article [2]:
> There is a way in which music can make a difference to your IQ, though. Unfortunately it requires a bit more effort than putting on a CD. Learning to play a musical instrument can have a beneficial effect on your brain. Jessica Grahn, a cognitive scientist at Western University in London, Ontario says that a year of piano lessons, combined with regular practice can increase IQ by as much as three points.
So schools at least push this narrative.
That said, I don't think teachers can argue in good faith that learning a particular will make you smarter versus learning another. Are multilingual people more intelligent than monolingual? Are people fluent in Ruby smarter than those fluent in PHP?
A better argument would be, learning music could potentially unlock the ability to learn new languages. Debatable, but I'd agree with that.
There is a difference, though, between learning something as a hobby, which is what most people learning music do, and mastering an instrument, which is a lifelong pursuit. The pursuits different, and so are the rewards. Learning something as a hobby will provide benefits, but none that can be associated with gaining more intelligence.
A better question is: why do you learn or teach music?
Learning music provides plenty of benefits for the development of children, but nobody teaches music with the specific goal of rendering people smarter.