When you make a mistake, the mail is not delivered. In addition, you receive a note from the electronic postmaster with lots of additional information which notifies you of this fact. The original message is included as part of this process. This is wasteful. The Emoticon Be friendly.
Writing is a one-dimensional process.
This leaves plenty of room for misunderstandings - especially when attempts are made at humor or irony. Someone invented the 'smilie'. It is now firmly embedded in the E-mail culture. This baby picture is inserted after a sentence which you want to be taken light heartedly. I use it all the time.