The linear composition is to make the cable housing compressionless. That's necessary for index shifting.

You are correct in noticing that at least some of the Jagwire brake cable housing also uses the same linear composition. If that wasn't reinforced by the kevlar sheath the greater cable tension used during brakeing would cause the cable housing to blow out at the curves.

Personally, I much prefer the compressionless Jagwire brake cable housing for use on mechanical disc brakes.