The WebKit2 GTK+ API is centered around the upstream WebKit2 interfaces, which is a new API supporting a split-process model and other incompatible improvements over the original WebKit API.
GNOME's Epiphany web-browser as of version 3.8 was ported to using the WebKit2 GTK+ application programming interface while other applications are still being ported. The benefits come down to the multi-process architecture, better robustness and responsiveness, greater security, and other new features.
Other benefits of the WebKit2 API include an embedded HTTP authentication dialog, support for GTK+2 plug-ins like Adobe Flash, a web inspector, accelerated compositing by default, and more. Some of the future plans also include multiple web-processes support, sandboxing, and network process.
More details on WebKitGTK+ 2.0.0 can be found from the blog release announcement.