There are several reasons for a pre park opening breakfast reservation. One is to get a head start on lining up for rides...most of the time you are not allowed to ride early, however, when BOG began doing breakfast, many times people were allowed early rides on 7 Dwarfs Mine Train.
We like doing them, particularly at Animal Kingdom, because seeing the park that empty is a special experience. MK is nice too, but there are more early dining options, tours, etc, that there are more people around. In my experience at Animal Kingdom, it's almost like you have the entire place to yourselves.
Most breakfasts take about an hour to complete, and you're sometimes out of there in advance of the crowds coming in from the tapstiles at the front gates. If you're finished earlier than that, you have the opportunity to get some nice photos and enjoy the quiet beauty of the parks.
Each family will have their own reasons, but we enjoy the low crowds of early morning
