Im so tired of having to decipher mispelled words that can change the meaning.. It gets annoying and perplexing.. Im not saying that they are dumb.. It's just that it would improve communication..
1) foreigners that try/know how to speak english but dont know how to spell and what not.. The spell check can correct the words and make the sentences more legible.
2) People who have spelling issues
EDIT: thanks to beat for letting me know that some browsers do spell checks but still adding it could help..
So maybe it can be an option that can be turned on and off.