Why does anyone (tech savvy or otherwise) need to bold or italicize anything.
In my mind bolding/italicizing are nice-to-haves alongside emoji. You wouldn't compromise the entire functional user experience of the central feature of your app for nicer emojis. (at least I hope not).
Please reread my comment. Italicising is, as I said, a nice-to-have. I did not need to italicise need; it's an optional extra in my comment, but I can still communicate effectively without it.
Or, as your reply has highlighted, even with the additional emphasis, people will still fail to read it the way it was intended.
Honestly, the unsatisfying answer is simply because they (i.e. those of us who didn't grow up on IRC, like marketing teams) want to, and are used to being able to do it elsewhere. I'm sure the people who include images in email and set their own fonts and stuff are happy with the upgrade; I can especially relate to that since I used to do some marketing and sales work myself.
I also hate the new input field but I understand why they turned it on globally. But a strong enough want is often indistinguishable from a need.
I myself use them sometimes to emphasize certain parts of the message. They can be very helpful. Many people like bolding certain words in an email as well to emphasize them.
In my mind bolding/italicizing are nice-to-haves alongside emoji. You wouldn't compromise the entire functional user experience of the central feature of your app for nicer emojis. (at least I hope not).