Some people use them to be mysterious or threatening, and of course, they can be massively slutty. It happens when two not-that-good friends find themselves stuck in the mutually-obligated chore of writing long descriptions of their lives to each other every few months.
Both parties dread having to answer all the last email’s questions and write a lengthy life description, and each is pretty bored by the process of reading the other’s.
With in-person interaction, we have a million subtle ways to express tone.
Even on the phone, without the use of facial expressions or mannerisms, tone of voice gets the job done sufficiently.
And with really close people, you can just skip the greeting altogether—no one starts an email with “Hey Mom”.