Replication is useful in in improving the availability of data.
This can improve availability remarkably because the system can continue to operate as long as at least one site is up. It also improves performance of retrieval for global queries, because the result of such a query can be obtained locally from any one site.
The disadvantage of full Replication is that it can slow down update operations drastically, since a single logical update must be performed on every copy of the database to keep the copies consistent.