What is purpose of Balun transformer in RF front end
Without much information about the signal connected to the input it is hard to say. However, one of the main applications of baluns is the rejection of common mode signals. In you schematic, since both connectors are expecting to receive the "same" RF signal, it seems that the balun is being used to attenuate common mode signals , e.g. noise.
The common mode signals which are in phase in both lines, will cancel each other out through the opposite magnetic field generated by the balun.