NoStore in Caching

I have been reading a lot about caching and had a question about the function of the NoStore property.  The description on MSDN is rather inadequate ("A value that determines whether to prevent secondary storage of the cached information.").  When would this be used?  Is this only for personal data such as username, etc.?  Is this something that would be useful for speeding up requests?  I guess I would just like a little more information about when this is used and what the default value is?
Thanks in advance!