Reverse proxying is a technique of storing the replies or resources from a web server locally so that the subsequent requests to the same resource can be satisfied from the local copy on the proxy server, sometimes without even actually contacting the web server. The proxy server or web cache checks if the locally stored copy of the web document is still valid before serving the cached copy.
The life of the locally stored web document is calculated from the additional HTTP headers received from the web server. Using HTTP headers, web servers can control whether a given document/response should be cached by a proxy server or not.
Web caching is mostly used:
To reduce bandwidth usage. A large number of static web documents like CSS and JavaScript files, images, videos, and so on can be cached as they don't change frequently and constitutes the major part of a response from a web server.
By ISPs to reduce average page load time to enhance browsing experience for their customers on Dial-Up or broadband.
To take a load off a very busy web server by serving static pages/documents from a proxy server's cache.
