Ask HN: Do Not Reply to This Email Why Not?

I think this question worth asking on HN because... it has been bugging me for awhile (so probably others too), there are people on HN who like to ask "why not" pretty regularly, and the answers may be helpful to founders/sales/customer support folks.

Why do companies send emails from "do not reply" email addresses?

At my company, we send onboarding emails, confirmation emails, notification emails, and more from our help email address and we tell people to reply to the email if they have a question.

What are valid reasons for sending an email from an email address that won't/can't receive replies? What am I missing?