I don't either! But this from the Apache directive you linked to:
The ETag value is used in cache management to save network bandwidth.
I'm inclined to think that it will be up to each browser/proxy to decide if or how to use ETags. Perhaps some "conventions" will or have emerged as to their use but I have no idea where to look or how to find them.
From your other link:
An ETag is an HTTP response header returned by an HTTP/1.1 compliant Web server such as Apache 1.3x.
ETags are "new." They first became available with the HTML/1.1 spec.