There are several reasons why more than one area is used in an OSPF network:

To improve performance. In some cases, dividing the network into areas can improve performance by reducing the amount of routing traffic that is generated. This is because OSPF routers only need to exchange routing information with routers in their own area.

To improve security. By dividing the network into areas, it is possible to restrict the flow of routing information between areas. This can help to improve security by preventing unauthorized access to routing information.

To isolate critical networks. For example, a company might choose to use a separate OSPF area for its critical production networks. This would help to protect the production networks from routing problems that might occur in other parts of the network.

To reduce the size of the link-state database (LSDB). Each OSPF router maintains a copy of the LSDB for its area. By dividing the network into areas, each router only needs to maintain a copy of the LSDB for its own area. This can significantly reduce the memory and CPU requirements on each router, especially in large networks.

To separate different departments or business units. For example, a large company with multiple departments might choose to use a separate OSPF area for each department. This would allow each department to have its own routing policy and would also help to improve security.

There are several reasons why more than one area is used in an OSPF network:

To improve performance. In some cases, dividing the network into areas can improve performance by reducing the amount of routing traffic that is generated. This is because OSPF routers only need to exchange routing information with routers in their own area.

To improve security. By dividing the network into areas, it is possible to restrict the flow of routing information between areas. This can help to improve security by preventing unauthorized access to routing information.

To isolate critical networks. For example, a company might choose to use a separate OSPF area for its critical production networks. This would help to protect the production networks from routing problems that might occur in other parts of the network.

To reduce the size of the link-state database (LSDB). Each OSPF router maintains a copy of the LSDB for its area. By dividing the network into areas, each router only needs to maintain a copy of the LSDB for its own area. This can significantly reduce the memory and CPU requirements on each router, especially in large networks.

To separate different departments or business units. For example, a large company with multiple departments might choose to use a separate OSPF area for each department. This would allow each department to have its own routing policy and would also help to improve security.