We3D can be downloaded for free by anyone on the App Store and is used to read on iOS any application created with Valkyrie Engine in a vpk format. It can be used to test out the applications you created on iOS before turning them into standalone mobile apps, or to distribute your application to your own customers.
Think of a vpk file as a PowerPoint file, you can redistribute it in the same manner. We3D is the application you use to read the file on mobile devices.
On a side note, on desktop you can publish directly in an exe format for Windows or use the Valkyrie Player which is bundled with the free download of Valkyrie to read the vpk.