Fat and Phat have two different meanings though.
Fat means well fat as in a fat person.
Phat means cool, as in that tune was phat!
Same with hot and hawt.
Hot generally tends to mean temperature, while hawt generally tends to be about a females appearance (I was careful with my words there
).
Correct me if I am wrong but it is all about subtle differences in meaning which is normally conveyed using your tone of voice. Obviously this is not available so it must be conveyed using spelling.