A long time ago, in a... never mind. Let's just leave it at "a long time ago" (I feel old enough as it is), there were people communicating with computer systems using dumb terminals (really dumb) and some computer systems couldn't be bothered with echoing back the character you sent - so you never had any feedback it was received. And so,
local echo became an option that allowed the dumb terminal to respond for the "couldn't be bothered with" computer system.
Point being, users want
feedback when inputting a command (whether it's text, mouse, touch, etc). Typing blind gets old fast the first time you drop your touch-typing hands onto the wrong row (for instance), not to mention having that hidden object also receiving commands from another input device.
Bottom line: there's no way in the OS to type in even a partially hidden window, although you can sometimes
select something from a menu or drag/resize the window using cmd-click or cmd-drag. This can be useful in repositioning a background window while leaving the foreground in the foreground.
As an extension of the above, perhaps this will help: you can also
cmd-scroll the background window, allowing you to peruse a web page, while leaving your Pages document as the foreground window. This would let you partially cover Safari with Pages, while still being able to read and scroll through an open web page. Just hover the cursor over the backgrounded window, then cmd-scroll (you have to use the multi-touch command on a trackpad or magic-mouse (or scroll-wheel mouse)).
Edit: You don't need to use the CMD key when scrolling a backgrounded window. Just hover, then scroll.
