Understanding Themes in general

Hi,
I’m working through learning WP and my question is about Themes, I think I’m missing some basic understanding here.
All Themes are editable (obviously), indeed infinitely editable. So why bother creating or searching through so many?
They’ve all got a header, which everyone will edit to be their own way. They’ve all got a background image, which everyone will replace with their own. They’ve all got a menu, which… well, you get the message.
So, in effect, is a Theme just about getting a layout or grid design that’s as close to what I’m looking for so I’ve got less editing to do?
Also, it seems to me that WP Themes are only 1 page (as opposed to, say, downloading a HTML design from somewhere like OSWD.org where most have several pages in their design) so I’m left to create the format/layout/grid design (sorry, not sure what it’s actually called) for subsequent pages anyhow.
Am I misunderstanding the concept of Themes or am I expecting too much?
Thanks