This additional interface will correct some minor vignetting that occurs because of the way the light rays are bent in the barlow. It helps to bring all the light to the eyepiece. In a separate issue, I have noticed some Kidney Bean black-out areas when barlowing the 35mm. In summary, the Televue Panoptics offer some of the best, sharpest views available, at medium to low power, along with a wide field of view. Stars and planetary images are very sharp.
Eye relief is for the most part comfortable, and light throughput is acceptable.
Some scopes will do better with the 35mm than others, and also the view barlowed reportedly varies with scope. If you had to choose only one, I would pick the 27mm Panoptic.