Maintaining a consistent, established schedule is an important step in supporting a child's normalization. Normalization is a natural process of human development wherein a child learns to fit into society.
A normalized child will demonstrate: a love of work (choosing an activity and working freely on it), concentration on work (focusing on the activity), self-discipline (following through with the activity), and sociability (respecting the work of others.)
When children know what to expect they feel confident and secure, and ready to absorb themselves in the normalization process.
The article, Managing Daily Transitions, provides more information about the importance of consistent routines in the Montessori approach.