In my case it's convenience. I have Telegram open most of the day and occasionally I use bots for small tasks that could be done elsewhere, but bots offer a more streamlined experience.
web: open browser -> find website in bookmarks -> copy email -> do whatever you need to do with it -> switch back to the website to check for emails
bot: open telegram -> find bot in chat history -> tell it to generate a new address -> copy it -> do whatever you need to do with it -> switch back to the app to check for emails
The only case I could think of a bot being useful is for semi-permanent email. ie. using the same email address for weeks/months, rather than one time only.