I addressed how barlows and focal reducers work in my post on focal length. Summarizing, they allow changing the otherwise fixed focal length of a telescope, which has an impact on magnification and or field of view.

Why would you want one a barlow

There are multiple reasons why you would want to have a barlow, but they share the same :

• Barlows allow you to reduce the number of eyepieces you need. 6 eyepieces can be replaced by 3 eyepieces and 1 barlow.
• Observers have the flexibility to change our eyepieces to change focal and hence magnification, astrophotographers do not have that luxury. Camera’s have whatever focal length they have, and changing the magnification can only be done through barlows and focal reducers.