An RDBMS eliminates redundancy. 

Lets look at the phonebook example. 

The phonebook represents a table that stores the names, addresses, and phone numbers for everyone in the city. 

Lets suppose you obtain a different phone book for the same city.

The second phone book includes email and website addresses along with the other data. 

Now you have two separate tables that store much of the same information.

Although the second source includes data not available in the first.