1. To make money:
This one seems to be the main reason for the creation of music videos. The music video is basically a promotion for the song, the artist and the album, because all three are represented in the video. Creating a music video increases the target audience's chance of hearing the song, and so increases the chance of the target audience buying the song. This means that the company makes more profit, which is of vital importance to the record company.
2. To give a deeper meaning to the song:
A music video can help to give a deeper meaning to the song, especially if they have confusing lyrics. Conceptual videos usually do this the most. Symbols shown in the video can reflect and complement the lyrics, which can help the audience to understand the message of the song. If someone doesn't understand a song, they might not like it; having a music video give it more meaning or to help the audience understand it may actually sway a person's opinion on it.
3. To entertain:
This is the purpose of nearly all media. The audience watches music videos, films, TV etc. to be entertained. They want to escape for a little while in the reality of whatever they are watching. However, it would be quite naive to assume that this is the main reason for the creation of music videos. There are artists who make music just for the love of it; however, it is also a job, and therefore they need to make money.
I'm very cynical and can link all three to making money, so I am going to say the main reason behind the creation of music videos is money and profit. The music industry is a business like any other and it needs to make a profit to continue. A music video is just a medium to advertise a product.