Spell Check And Proofread Messages
Spelling is a part of proofreading. The most important part. Given, we can usually get the message if we understand the words. So, we have to spell them right in the first place.
Proofreading is more in depth. Requires that we reread the entire passage. On the other hand software can check spelling and grammar. But, does not tell us if we used the wrong word.
In any event, we shall send messages we approve of. Only way to know is to check. So, we review before sending. Hence, the rule.