BALUNs are in general deployed at transitions between a-symmetrical to symmetrical transmission line environments i.e. where a balancing function and sometimes also an impedance transformation function is required.
This is the case where transmission line has an a-symmetrical and the load has a symmetrical nature.
Typical usage is feeding dipoles and yagis.
Common-Mode Choke type Current BALUNs have many advantages above traditional voltage type BALUNs.
They ensure that the RF current only flows where it is wanted. (i.e. on the inside of the coaxial cables and in the antenna elements).