Q. Why are old non-fiction books still kept in the library, how are they useful?

The value depends on a variety of factors. 

In some cases, older information is just as relevant (art books for example).  Additionally, older items can serve as a historical perspective or be used as a comparison to current conditions (histories, of course; or the state of the Internet in the late 1990s vs. today). 

Occassionlay out-of-date items are "weeded" from the collection.   Weeding is the process of withdrawing unneeded and outdated materials and removing them from circulation.