The purpose is to get an estimate of the available image size and how it is limited by image quality and vignetting. The normal adapter plate supplied has a thread for Schmidt adapters. The aperture in the Schmidt thread adapter is 38 mm. The large aperture on the rear of the tube is thus greatly constricted by the normal Schmidt adapter opening. This is because the Schmidt thread, while somewhat standard on the back of SCTs, was originally designed for small telescopes.
Several manufacturers provide attachments for the larger telescopes which have a larger apertures.
The JMI focuser can be obtained with an adapter that has a 51.2 mm aperture. Their focuser itself has an aperture of 50.6 mm. The aperture on a 2 inch eyepiece is slightly smaller. Generally 48.5 mm. A 48 mm filter has a clear aperture of 43 to 44 mm.