Oh. That I can explain a bit better: It's easier to read. IF everything was a neutral color like right now, it gets sort of hard to keep track of what's being said and done. If [b]Bold[/b], for example, is being used for dialog than if you should respond to me you can just glance at my post and see what my character is saying instead of having to track down the quotation marks and trying to keep up with it, especially if I get into long winded explanations that dip in and out of dialog. It also helps make it clear what certain dialog is. If I speak in [i]Italics[/i] that means I'm thinking, and your character shouldn't respond to it unless he can read minds. Using both [b][i]Bold and Italics[/i][/b] is often just used for dramatic effect, but generally I consider it to be spoken dialog unless otherwise informed.

Some circles also see it as a sort of placement test as an RPer, as most new Rper's don't know the code well enough to use it in such a matter.