When you're in Safari, you use one finger for scrolling, as you know. But that always scrolls the entire viewport, not the page itself. That is, the small rectangle you're "looking through" to see the webpage is what moves.
Okay, but what if a webpage has a frame, or other scrollable subsection? On your desktop, there'd be scrollbars within the page that you'd use. But oh no, the iPhone can't be seen with those. So to scroll a scrollable subsection instead of the whole window, you use TWO fingers at a time. The multi-touch screen sees that.
The reason the question often comes up because when you tap on a scrollable area to enter text, then the keyboard covers up a lot of the area. Plus you type enough, then you scroll off what you were replying to. Very frustrating, unless you know that you can dismiss the keyboard and then use two fingers to scroll up and down again.
Not sure I'm explaining this well.