Cache-Control no-store, no-cache in Headers. Why?

Hello!

Looking at logs of my local proxy I found that my site responds with "200 OK gzip" for an anonymous user. It's completely OK but I was hoping to see "304 Not Modified gzip" when the user loads the same page again (if it's cached of course).

However I was always seeing "200 OK gzip". This means that cached page is always retrieved from server even if it was not modified.

Trying to find the reason I noticed that the pages were never saved to browser's cache and in the end I saw the following:
$string .= " Header set Cache-Control \"no-store, no-cache, must-revalidate, post-check=0, pre-check=0\"\n";
In boost.module and in cache dir .htaccess

When I removed no-store, no-cache it started working as I expected.

I thought that these directives were added to prevent the user from seeing an outdated content. However, if I delete a page from a cache it is served to the user instantly. So this is not the case.

And the question is - "What is the reason for no-store, no-cache?"