This feature is already available to some users. I haven't seen it in any of my five Google accounts, but I hope to have it soon. You can check if you have the feature by simply opening any multiple page Google Document that you own and then right-clicking on a page to look for a menu that allows you to choose a page layout for a section of your document. Take a look at Google's announcement of this feature to see an animated GIF of the new layout menu.
Applications for EducationBeing able to mix portrait and landscape layouts within the same document could prove to be helpful to students and or teachers who want to include landscape-oriented tables and charts in a document without having to write the entire document in landscape mode. For example, I can think of a few documents that I use in my networking course that would benefit from having diagrams in landscape mode instead of the portrait mode they're in right now.
On a related note, if you've ever wondered how to change the default font on your Google Docs, follow the directions in this video.