for breakfast, many prefer to have an early ADR. CP usually opens at 8:00 AM even on EMH mornings. It may open earlier on very busy days but I don't remember ever seeing that happen.
The main reason people ask for the early time is to get into the park when it's relatively empty and get pictures at certain points. You can't roam the parks for a long period of time but pictures in front of the castle and by the Walt Disney statues are pretty common. For that reason days without morning EMH are more difficult to get than those with them for those very early times.
A second reason that I think it's a good idea is that you get through the meal a little quicker. I find that at the larger character meals the meal length is driven by when all the characters get around to your table. For CM and CP that's often 75-90 minutes. Getting in at the beginning cuts that time down if your anxious to get into the park.
