Face detection is used to know how to position your photos within their containing frames, and also to know if a photo contains a group of people, such as in a wedding group photo. When a group photo is detected, it is placed on its own page, without other photos.
Note: the green rectangles are simply there for illustration purposes.
When using the advanced wizard, setting the "Enable face recognition" option lets the wizard create an album which is better suited for a wedding album.
When placing images into an image frame, any faces detected will be panned into view according to their relative distance from the edges of the original photo, to match the intention of the photographer...