Individual students learn in different ways.
Manipulatives can be used to represent both numbers and operations on those numbers. In addition to meeting the needs of students who learn best in this way, manipulatives afford the teacher new ways of visiting a topic. A sound lesson on any mathematical topic should involve multiple instructional methods. Incorporating several different instructional techniques increases the possibility that all students will develop mathematical understanding through at least one method. When I think in terms of varied presentations of the same problem, I can't help but allot more time to the material.