I'm clear about why we need to convert to IF at the receiver side before down covnerting to the baseband. but why do we need to do it at the transmitter before upconverting to the RF frequency for transmission purposes???
Please understand the important basic thing is we are downconverting or upconverting the carrier only not the signal. We cant change the frequency of our signal. Moreover in IF frequency range we can easily achieve linear performance of the devices (what we are using for processing ) than RF.
In transmitters it is frequently more convenient, cost effective, etc. to generate and band limit the modulated signal at a lower frequency. It can then be mixed up to the final frequency. With low cost DDS and multiplying PLLs the mixing to the final frequency can be done economically.