"The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it."
Terry Pratchet, Diggers
Material Design Resizer by Google - link
We believe this is a good tool for your clients to preview or test their websites during UAT or before going live. And while its specifically designed for the Material layouts, it definitely does not mean it will not be suitable for a site you're building for a client.
"But what about good old manual browser resizing or the responsive mode via developer tools?"
They definitely work too. But from experience, most of our clients are not technically proficient enough to be comfortable using those tools. The Resizer website tool feels much more "professional" and it allows users to easily click on the desktop or mobile tabs, or switch between the pre-defined widths. And, its by Google - so again, this is something they *may* trust more.
But of course, if you're a developer, you should be coding your responsiveness based on more than the predefined widths available there.