From Google's standpoint, Web Designer is a way to cut through the confusion about different platforms, device types, screen size, and other factors which can get in the way of advertising online. "There are too many choices to make when developing for mobile" the company argues, "too many platforms, browsers, and devices to build for, and no tools that simplify the development process."
Into that ecosystem comes Google Web Designer, described as a "professional-quality design tool" that doesn't require graduate-degree skills in coding or design to produce animated content. Once they're created, Web Designer can push them straight into DoubleClick and AdMob campaigns.
However, advertisers aren't likely to be the only user-base finding a simple animation package appealing, especially when it doesn't cost anything to download. Just as GIFs were co-opted from simple online adverts to complex film cuts and more for microblogging services like Tumblr and Twitter, so Web Designer is likely to find appeal among those looking for animations with minimal headache.
Those animations can be quickly pieced together at a scene level, or alternatively there's an Advanced mode in which each of the individual components, transition times, easing, and other factors can be adjusted manually. That all happens on a timeline with multiple layers, the latter just as you might find in Photoshop or similar.
Of course, Google Web Designer won't know its hit the big time until a full animated movie is created with it. There's information on how to use the app here, and a demo video below.