There is one big problem with using music in videos, and that is that music is protected by copyright and you are not allowed to use any of it without permission from the composer or songwriter or it's legal representative. The right music can draw people in, and create the type of emotion that you are aiming to create with your videos, resulting in more views and higher ad revenues. Posting a video on YouTube without any background music is just not on, so using music in your videos is a very important part of your success. What you should know about using music in YouTube videos
What kind of music you have to use and how Royalty-Free Music for YouTube can help. How to avoid copyright issues when using music in videos posted on YouTube.