What if we cannot predict lifetime of page content? If we have a page with info that changes unpredictably we still can use browser cache to avoid unneeded traffic.
Using validation mechanism browser sends HTTP request with info about cache entry and server can respond that the content wasn’t changed.
There are two validation methods: one is based on Last-Modified and the other is based on Etag.

Last-Modified
Server sends Last-Modified header with datetime value that means the time when content was changed last time.

Cache-Control: must-revalidate
Last-Modified: 15 Sep 2008 17:43:00 GMT

The first header Cache-Control: must-revalidate means that browser must send validation request every time even if there is already cache entry exists for this object.
Browser receives the content and stores it in the cache along with the last modified value.
Next time browser will send additional header:

If-Modified-Since: 15 Sep 2008 17:43:00 GMT

This header means that browser has cache entry that was last changed 17:43.
Then server will compare the time with last modified time of actual content and if it was changed server will send the whole updated object along with new Last-Modified value.

If there were no changes since the previous request then there will be short empty-body answer:

HTTP/1.x 304 Not Modified

And browser will use the cached content.