To promote and advertise the song, album and artist.
It may provide a visual representation and perspective of the song
meaning and or story.
Back when MTV started out, they couldn’t always bring in the artist to
promote and perform their song, so instead bands and artists alike would just
send in their music video and MTV would than instead show that to their
viewers.
Having a music video also increases the production value of the song
and gives the audience/consumers something else to react to, overall making the
song more memorable.
They could also be used to promote other Medias such as films and video
games. Possibly showing scenes from the film or gameplay footage from the game.
Eg, Fast and Furious 7.
A music video also helps set up and establish a band/artists’ image and
portfolio.