Microwave transmission
why do we need transmission lines(conductors) to transmit microwaves when we already know that Microwaves can be transmitted wirelessly?

Why do we have coaxes connected to parabolic reflectors (LNBFs)? When transmitting, the coax and subsequently, the feedhorn, feeds the reflector with the intelligence (encoded, modulated, and power amplified signal) which is focussed onto the reflector by the feedhorn for transmission towards the target. So, a transmission line connects your transmitter module to the transmitting antenna, which is the final stage of the transmitter taken as a whole.
You are correct- microwaves are indeed thrown off and received by antennas (aerials). There are no physical media such as ethernet, coax or a fibre optic cable that is present in between the transmitter and receiver to carry the microwaves. Transmission lines are used to feed the antennas and there must be impedance matching between the transmission line and the antenna that it feeds.