I'm guessing you mean an extension. Thought about it to be honest, the only thing I find them good for is being a rear sunshade. Not for me, never really used one on my set ups, but some do for the purpose above, or as an alternative to 'personal' shooting.
With this I mean confining yourself to the shot. In hft, I tend to put my hood up, like in the image I have, before getting down to the peg to block out what's going on around me... They possibly do that on a smaller level.