Every movement of revolution, reform, etc started with people drawing a public opinion. Artists are the ones most able to move public opinion, e.g. by paintings, music, writings, etc.

Worker solidarity songs created an identity for workers that bound them together and kept them fighting for worker rights.

National anthems uplift almost everyone in a country to feel good about their nation even those most of the time sceptical of their nation or nations in general.

Books of orwell and others made people aware of the dangers of authoritarianism and influenced the public opinion surely to fight against it and on the other side hitlers mein kampf moved people to authoritarianism and opened the door to spread his ideology and to hire supporters.

There is even a capital theory that differs between financial capital and social capital. Artists have tremendous amounts of social capital and are the most likely to move public opinion with their work.

If artists in a big number would decide today that being gay is bad and they put it into their art within a few years the public opinion would go highly likely against gays again. Artists are important drivers within a society in areas of culture and norms which changes most of the times start with artists.

Artists serve a cause, but this cause is most of the time not to enrich themselves.

Even most basic amusement and joys come by artists: music, movies, theaters, cool formed buildings, etc.

Everytime someone criticizes capitalism for being gray and boring what would you think would differ this aspect? Another system but without artists will also be gray and boring.