Music is what sets the tone of a slideshow. It is an important part of the process in creating your slideshow. Choose songs that will complement the overall feel you are aiming for.
If you are creating a Birthday Slideshow for a child, you will want to use music that is upbeat and fun. For an older persons, you will want to use music that matches the time period, or their personality, or maybe their favorite song.
For a wedding slideshow you will want to use fun and upbeat songs for the photos of the bride and groom growing up. Then switch to more romantic music for the pictures of the couple together.