there are a few reasons I can think of to use tiny type. For one, designers who are used to print might not understand that things should be bigger on the web, because you generally read websites at a further distance. Second, most fonts aren't designed for screens; and thus don't look good at larger sizes. 18pt for a font designed for body copy in print can be jarring sometimes. Lastly, and this might sound silly, but smaller type leaves more room for other stuff. Some people think scrolling is a pain.
