This device is really more than just an optical filter. Certainly the filter is a key part of the assembly, but the convenience of having it housed inside the adapter that attaches to a standard Canon EF or EF-S mount on the camera side and with M48 threads on the other side for attaching to the telescope should not be underestimated. If nothing else, the filter helps to ensure no dust can get inside the camera body if the adapter is kept attached to the camera when not on the scope.

The image on the left shows the holder fitted with the Baader Planetarium UHC-S filter, which is a type of broadband light pollution filter but also enhances the contrast between emission nebulae and background sky by ensuring high transmission of the H-alpha and OIII wavelengths. The filter can actually be swapped out of the housing for different filters, and I have both the UHC-S filter and a 7nm bandpass H-alpha filter.

The transmission spectrum of each filter is shown below.