It adds fun to formality - Though your event may already be a low-key and more casual, there is always need to break the ice with a large group of people who may not be familiar with one another. The instantaneous laughter and relaxation that photo booths create serves as the ultimate icebreaker and adds an element of fun that no other event activity can deliver.
A great gift for guests - Most photo booths offer traditional photo strips, which your guests get to take home with them. This can replace the line item in your wedding budget allotted to favors for guests, adding to the fun and letting them have a wonderful memento of the wedding.
Far more photos - Even if you have a highly skilled photographer who relies on the help of a second shooter, you are not going to get the same number of photos from them as you can when you add a photo booth to the mix.
It can add to your theme - Most event use at least a color theme or general concept (top hat formal, country casual, etc.). Whether you have a basic theme or an over the top theme (such as superheroes or a favorite film that inspired the colors, décor, and even garments), the photo booth can reinforce the theme. Props and backgrounds can be in total alignment with the theme and emphasize it in your photo albums (real world and online).