For some people, the command you gave would fail because they can't bind port 80 because apache or nginx is already listening there. And they might not be able to stop it and restart it. That's not the case if you're only running an SMTP listener, but it would be if you're already running an HTTP listener.

(I wrote the original version of standalone in certbot, but I'm just trying to stand up for the people who complain that something is complicated in their use case.)