Rails - etags vs. page caching (file cache)

Question

What would be some advantages of using etags/stale?/fresh_when? instead of page caching (on a file cache)?

Apache automatically handles etags for static files, but even

Answer 1

One thing that comes to mind is that fresh_when will still save you some rendering even if you cleared the entire page cache. Here you'd be using both in tandem.

I'm curious about other answers as well.