Japanese style inn, ryokan normally offers full board; accommodation including dinner and breakfast. Although there are many relatively small ryokan which offer without full board, Japanese guests prefer full board included reservation.
The reasons are - 1)There are not so many places to eat around the ryokan. 2)Many people come to enjoy the bathing and these people eat after having the bath. Also there are many group reservations which are not typical tourists but colleagues or former schoolmates. These people are looking for a venue which can accommodate large groups.
Dinner at ryokan used to be at your room but now is popular to have it together with other guests at the dining hall. Then, nevertheless you eat at your room or at the dining hall, mostly the facility designates the dinner time. Dinner at ryokan is relatively early, starts at 18:00 and usually finishes before 20:00.
Now many places offer buffet type breakfast as similar as hotels but there are many ryokan which offer traditional breakfast only. Due to there are many Europeans who dislike eating Japanese food in the morning, you should better check the breakfast details before making the reservation.