Later, the same data can also be used to get other information that was not needed for the first use. The store might want to know now what kind of things people buy together when they buy at the store. (Many people who buy pasta also buy mushrooms for example.) That kind of information is in the data, and is useful, but was not the reason why the data was saved. This information is new and can be useful. It is a second use for the same data. 