Caching is a good strategy to speed up a Joomla website. Not just Joomla but every content management system. What caching does is to save a static page and render it each time the page is clicked by a website visitor. Instead of loading the entire page along with all the extensions, what it does is to serve the saved page like a static page.

How does the web cache work?

Joomla wants to present the fastest user experience possible to the visitor. As someone clicks through the site, Joomla makes snapshots of where they have been and saves them to a temporary directory. If the person wants to return to a previous location, the server grabs the temporarily saved page, which should be faster than going back to the data base and generating an entire new page which will load all plugins and extensions.

Sometimes this cache doesn't get cleaned out soon enough, so when you make a change in the administrator area then go to the front end to view it, you're actually looking at an "old" page that hasn't been updated yet.

Read full blog post on Todhost website: https://www.todhost.com/blog/why-the-progressive-caching-in-joomla-should-be-avoided.html