Most corporate databases used today are relational in nature. You have tables, columns, rows, column indexes, relations between tables using associated columns, etc. All this means you are dealing with structured data. Think banking systems, mortgage systems, customer and order tracking systems. These systems are well served with relational databases.
Now think about the systems that a lot of us deal with nowadays. Unstructured or semi-structured data as available in Social sites (facebook), wiki, blogs, news sites,etc. This data is clearly not relational in nature. Instead they are document-centric in nature. While some RDBMS can store this type of data, its not natural for them.