What is the purpose of music? Is it just supposed to be entertainment or should it be a medium for social expression and change?

it is both

clearly teen pop serves it purpose

but the Beatles and Bob Dylan are huge as well.
jazz and swing are good at the whole sign of the times issue
and Billie Holliday with "Strange Fruit" defintely made a statement

there are few artists that can do both
be commercially successful and very entertaining
and also have a strong message

most fall into one category or the other
kinda like FunTrivia!

LordAndry, you didn't mention the Classics, Opera, Light Opera, Military and Brass Bands (the sources of most popular music! )
As Shakespeare said " If Music be the food of Love - Play on!" it's primarily for entertainment but can be a very powerful information or propaganda tool.

Music is like a language that reaches more people than any other.
It's more elemental than just the instrument for social change..it is both popular and that too..because it reaches more people than any other medium..honestly!

I'd say that any religion that tries to do away with music is doomed for failure.

Music can get by the barriers of language really...you don't need to be able to read to enjoy it..to hear it. You don't even need to understand the language if it's a song.

The smallest child, yes even the foetus can appreciate it...I knew that when I played while expecting..so, it's the first language.

There are many purposes of music and they vary according to the artist:

--To provide entertainment
--To express points of view, political opinions, etc.
--To arouse emotins that inspire or motivate people to think or do certain things
--To offer praise or worship in a religious context
--To express feelings of love, patriotism, or anger
--To provide complement to movies, theater, dance, etc.

I do not think there is any one thing music "should" do, but at its most basic level, it at least has to entertain, or no one would want to listen to it.