What's up with Wat and Wen?

I understand using abbreviations and slang on the Internet; I know all about LOL and so forth. But why do people use "wat" instead of "what" or use "wen" instead of "when"? I can't see how leaving out a single letter really saves time typing! I also see people using Z's in place of S's, or spelling the word "does" as "dose." It doesn't make any sense to me--- there is not a big reduction in the number of keystrokes, and it's sometimes really hard to figure out just what people are asking. Some folks look like Hooked on Phonics rejects!

Can someone please explain it to me? Thanks! I have been wondering for a while now!