Large OSPF (Open Shortest Path First) networks use hierarchical design for three main reasons: to speed up convergence, confine network instability to one area, and to reduce routing overhead. Hierarchical design basically breaks a network up into smaller portions making them easier to manage.
This design was first submitted by Cisco in 2002 as a best practice. There are also other benefits to using a hierarchical design such as better security, cost savings on equipment, traffic multiplexing, and relative ease of design. Each of these benefits can be related back to the three main reasons in using a hierarchical design.
To confine network instability to single areas of the network. 2. To speed up convergence 3. To reduce routing overhead
Hierarchical design of OSPF (basically means that you can separate the larger internetwork into smaller internetworks called areas) helps us create a network with all features listed above (decrease routing overhead, speed up convergence, confine network instability to single areas of the network).