One of the reason for providing this binary packages is for users who wanted to help MATE devs testing the GTK+3 build as it will be the future of MATE development. As we can see from several MATE release, more and more MATE components are now being ported to GTK+3 only with a long term of fully supporting GTK+3. By removing old GTK+2 code, the developer can finally implement new features such as:
- Wayland support
- HiDPI support
- Migrate libunique to GtkApplication
Note that GTK+3 is still considered experimental, but the future of MATE will fully utilize GTK+3 so it's a good idea to test them early. By the time next Slackware release is available, i believe we are ready to fully use GTK+3 build.