The purpose of the Barlow would be to increase image scale. A 2x will double your focal length and focal ratio but also considerably increase your exposure time. It is useful for say the Moon on a full frame camera. It is also useful on say an SCT for planetary imaging with a small sensor CCD or CMOS imaging camera taking many frames per second.
Normally at 1200mm focal length the 0.5 deg. Moon will give you a 10.5mm image on the sensor. A 2x Barlow will increase that to 21mm which fits nicely inside the 24mm short sde of a full frame sensor.
Note if you are contemplating imaging a 2x Barlow is fine but a better method is with a 2x Powermate and although you can use it as a standard Barlow with standard T-adapter the Powermate has an optional T adapter that is a better match for it to provide the correct amplification factor.
See
http://www.televue.com/engine/TV3b_p...d=123&Tab=_con
and
http://www.televue.com/engine/TV3b_p...d=123&Tab=_app
and
http://www.televue.com/engine/TV3b_p...d=123&Tab=_acc
Note you may also want to consider a wide aperture T-ring and T-adapter to avoid vignetting with full frame sensors. (A standard one will work fine if you want to experiment first).
See also
https://www.telescopeadapters.com/
specifically
https://www.telescopeadapters.com/94...format-t-rings and true ultra wide is
https://www.telescopeadapters.com/89...ters-for-canon This last one has integrated T-ring and saves about 10mm focus distance required. This would be for prime focus use and not with the Powermates for which I recommend their own T-ring adapter. Lots to choose from.
Enjoy!