Question

what is BJT in CC mode is used for?

Answer 1

bjt in cc mode i suppose enables us to have a high current gain so whenever we have to use a transistor as acurrent amplifier we use cc mode configuration

Answer 2

In common collector mode bjt acts as emitter follower.

Answer 3

CC config has output voltage same as input voltage (Vo/Vi = 1)
high input impedance and low output impedance .therefore it can be used for impedance matching purposes(matching high impedance source with low impedance load) .Its also used for isolation purposes.. to avoid magnetic coupling between two stages