Virtualmin and Cloudmin have CLI apps (imaginatively called "virtualmin" and "cloudmin", respectively) that can do everything the GUI can do (and a little bit more, since they can readily be wrapped up in pipes and for loops and can do things in batches very easily).
At some point, when the scripts are advanced enough, all you're doing is data entry. Data entry may be easier/faster in a web-based UI with dropdowns, radio buttons, multi-select boxes, etc. It is also easier to delegate to untrusted or partially trusted users when you can control every option they have available to them. Ever tried to limit someone to only being able to modify some DNS records, but not others, using sudo and filesystem permissions?
A primary reason Webmin is web-based is that a web browser is always available. An ssh client may not be. We do have plans to create a "webmin" CLI tool, at some point in the future, that does the same thing for Webmin that the virtualmin and cloudmin tools to...though that's a trickier business since Webmin isn't really like either of those tools, and is a lot more open-ended in its purpose and capabilities.
But, sure, command line tools are awesome, and Webmin doesn't prevent you from hitting the command line when you want to. I spend my time about 50/50 in Webmin/Virtualmin/Cloudmin and in vim via ssh.
Again, I'm not opposed to people doing their job any way they want. I'd just like to see an end to the FUD that only incompetent sysadmins use tools that make their jobs easier ("It has a GUI!? How dare they!"). I've spent my entire professional working life trying to make sysadmins jobs easier, so I want people to make informed decisions about the available tools, and I've almost never met a sysadmin that couldn't save some time or hassle by using one or more of the tools I work on. It's not the right tool for every job, and I don't recommend everybody install it on every server they ever see (though some sysadmins do), but it's got a lot of niches that it serves very well.