The Preallocation Pattern is a design approach in MongoDB that involves creating an initial empty structure to be filled later, balancing the need for simplification with the additional resources consumed. This pattern can simplify code and improve performance in certain scenarios, such as representing two-dimensional structures or tracking accessible seating. However, its use has been limited by the deprecation of the MMAPv1 storage engine, but it still holds value in specific situations, requiring a trade-off between simplicity and performance.