Data is what the Web site consumer is seeing and doing. It flows in and out of the database server. When you say that a Web site is slow, you really mean that it's taking a long time for the data to travel from the servers to the browser, or the other way around. Web application is likely to have many different kinds of data. Alternately, this data may be acquired from external data providers.
All data repositories have their own static data representation.
You need to reconcile and integrate these different data repositories so that they work together in a coherent and complementary manner. Once the data structures and data dependencies have been identified, you need to define and then design the data flows in the system. Or they may require building realtime interfaces to external data providers. The data architect must understand all the data requirements and design appropriate data repositories and data movement in order to fulfill all the business's needs.