Sometimes when I'm trying to be productive (it happens, I'm sure) I like to have two windows open, going back and forth between them. I often use one window as a reference for another window, say, like when I am writing a paper and need to read something in the browser as a reference.
There comes a time, however, when I need to scroll down in that non-active browser window. On Microsoft Windows (all versions I've tried, including Windows 7), this is not as simple as it should be and can be really irritating if working for an extended period. The user must click on windows to make it active, and then he or she can scroll down using the scrollbars or the mouse wheel.
On a Mac, windows are not required to be active in order to interact with them. If I'm writing that paper and reading a PDF in another window, I just hover the mouse pointer over that window and scroll—no clicking required.