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.
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.