To ease the burden placed on bandwidth. I am based in the UK and caching enables me to retrieve data located on servers in USA much faster because there is a heavy burden placed on transatlantic bandwidth. Most caches will then calculate the time since the document was last modified and store it in the cache for a proportion of this time. The default for the CERN cache, for example, is 0.1 times since the last modification time. The results of CGI scripts and other automatically generated data are usually not given either of these headers and are generally not cached.
Caches clearly can cause problems in that they can give out old information.
However, if your pages are not cacheable you may find that you lose huge sectors of your audience. If your web server is in USA then Europeans using caches will be able to retrieve your pages quicker if they are already in the cache. I can assure you that it can often take a long time to get data from USA to Europe if it is not in a cache and often people will just give up if they can't get the information they want quickly. Tell your clients to use the Reload button to get the latest version of a page, but remember that caching does have huge benefits for people who will view their web site. If a page is 10 days old it will typically only be cached for 1 day, which isn't really a huge period of time.