The fact is scrolling with a mouse moves much faster while still seeing what I'm doing. You can't scroll line by line quickly with a keyboard. There's no variable speed (that I've ever seen, at least). However with a mouse I can alternate between fast and slow. Really nice for scanning large piles of text where you don't know exactly what you're looking for.
Sure I could jump a screen length at a time if I wanted, but then my eyes have to jump bottom to top to bottom to top every time I jump. I find it difficult to track where I am. Page jumping tends to ruin context, even with a handful of lines as context.
With that said though, I have seen some really nice Terminal renderers that will smear the cursor when you jump. It looks like it would help a lot to keep the visual context.