What is Output Caching? When it is required for us?

Question

Output Caching:
     Its used to improve the perfromance of the asp.net application by the caching the entire page with specific period of time. It will get information from older version of the caching.
Example:
          <%@ OutputCache Duration="10" VaryByParam="none"  %>
The above example is showing outputcache directive with duration 10 min to cache the entire page.  The output caching is better for caching the entire page.