What is the purpose of a music promo video?

A music video can be made for many purposes including;

• To promote the song. For example if the song isn't very good or catchy then if the video is amazing the song may be more well known and an increase in popularity may occur. 
• Extension of income. So the company may want sales to increase so making a video means they can sell this separately and increase sales or get paid for people and websites to show this video. 
• Synergy. So the music video can drive the song into a style or genre which can be used in films and TV programmes.