Film is widely acknowledged as the highest art form since it in the very least constitutes of several different art genres. A film for instance could have music, be based on literary material, use imagery such as paintings to indirectly drive a message home and so on. This gives films a wider scale to out-do all the other art forms in terms of achieving the purpose for which they are intended.
From eons gone by, art (even at its crudest form as paintings when all man could do was paint pictures of animals on the cave walls where they lived -- to the age of CGI) could at the very basic level be used as a form of self-expression. People have the inherent quality of telling others of their achievements and this was one way to leave something for the other generations to see what their ancestors could do (which was not much).
Art, from then on advanced and with that the reasons for which it was created became more refined. Up till the 1950s, movies were predominantly made for entertainment but slowly the effects of the wars made people realize that films could do more than just entertain people. At first the power of films was used as balm to psychologically soothe people using synthetic mode of psychodrama which enabled audiences to release pent up emotions by watching others enact situations they were familiar with.
The success of this venture, encouraged filmmakers to tackle harder social issues which had either been ignored or personal matters which those involved in making the film wanted to be addressed and thus film became the number one social commentary tool. For a time this encouraged some change in society but not much of it. Nonetheless, once it was realized that there was a possibility of using films to actively influence people moviemakers chose to edge further into the psychological territory of their audiences.
This finally gave birth to the use of the medium as a tool for propaganda. From the cold war era, movies began to be used to subtly garner sympathy for the cause of communism or capitalism depending on which side made the movie. By presenting their enemies as contemptible people, the national populace who had no idea what the conflict was even about were misled to believe that the people on the other side were as bad as they were presented to be. That, far from making things better served to up the ante in terms of driving the tension a notch higher.
Much as propaganda was successful in painting the government's enemies as the bad guys it didn't satisfy movie-makers. A little success whets the appetite for more success. The movies were turned on the people themselves. Depending on the needs of the filmmaker, people could either indirectly be induced to watch more movies, in order to make the production company more money or used to support a movement such as feminism, gay rights, etc.
Over time the main purpose for which films were originally made has become a side issue. Films were meant to entertain people and not used as a platform for educating people, encouraging them to be good guys and so on. Yes it is permitted that a great script should integrate such things into it but then it was the stories' themes were meant to present themselves to the audience and not have the messages shoved straight into their faces.