Since all of the images in your galleries must be placed into categories, many photographers feel that it's easiest to have a set of predefined categories available with each of their gallery types rather than manually re-creating similar categories each time they create a new gallery.
For example, if you shoot a lot of weddings you might find that each of the weddings can be broken up into similar categories, such as: Getting Ready, The Ceremony, Bride & Groom, Reception, Cake Cutting, Candids, and Guest Portraits.
Any categories that are left empty (have no images in them) on a specific gallery will be hidden from your clients when they are viewing their images. Also, if you need to create additional categories that are specific to an individual gallery you are free to do that through the categories page inside the gallery itself.
So, whether or not you create a set of default categories within your gallery type settings is entirely up to you, but if you're shooting a lot of similar events it could save you a little time and hassle when compared to recreating the categories on each of the individual galleries themselves.