This is directly related to “responsive design”:
During the design phase and building for the various breakpoints (or screen-sizes, however you relate to that schema) I was under the impression I had to physically use my mobile device to check the results from a published site. Yes, you can the preview function in Sparkle, but that’s not the same as the site in the wild, real-world.
What I just learned is that if I squeeze the browser from wide to narrow (Firefox/Safari) I get to see all the 3 breakpoints I designed, PC-Wide down to Mobile. In fact if the browser is narrowed far enough I see exactly what the mobile version looks like on my phone, without having to use the phone.
I think it’s a neat and quick function to see how things look/work without physically fumbling around with multiple devices. No, this doesn’t replace actually using a mobile device, but it sure cuts down the post-publishing clean-up phase.