OT: Are Includes Loaded in User's Memory?

After searching the internet I'm unable to find an answer to what I thought
would be mentioned as one of the reasons to use SSI Includes.

Since one of the touted benefits to using CSS external stylesheets is that
they are downloaded once per session, which saves some bandwidth and reduces
the file size for subsequent web pages, I thought the same may be true of
Includes.

Is it possible that SSI Includes share the same benefit as external CSS
stylesheets in that the Include files need to be downloaded once during a
session? Or are they downloaded for every page that calls them?

Richard