We eat earlier. We stay in the parks from rope drop until early evening. After all that walking, we're beat and ready for an early dinner. Another reason is my girls want to have pool time at the hotel. We would rather have swimming time after dinner vs. middle of day so that we don't lose park time traveling back and forth from park/hotel. Plus, they can just take a bath after an evening swim and be ready for bed. Too much time lost on mid-day swimming not only in transportation back and forth & cleaning up before dinner. So that they have plenty of swim time, we set our reservations early. Also, my youngest will take a nap in her stroller while daddy takes oldest on rides. She wouldn't take a nap if we were swimming, so it's better for everyone's sake that she has a nap while I stroll her around. That forces swim time for the evening, which means we have to have the meal reservations earlier. We've had a full day of park activities, so we're ready to eat early anyway.
Even if I didn't have kids I still would set reservations a little earlier so that you can take advantage of light shows/parades, etc. better (stake out curb positions, etc.). I also would still tour the park throughout the day and leave pool/relaxing time of the evening which would require earlier reservations.