Role of Mixer post Modulation Stage
What I understand that in "MOD" level the actual signal is mixed with the carrier frequency

That is not the case, MOD stands for Modulator which is a block that applies a certain modulation to a signal. For example, FM modulation for an audio signal or OFDM for a datastream for WiFi.

At the output of the MOD you would then have a baseband signal. You cannot transmit this signal as the frequencies are too low. This is where the Mixer comes in, it mixes the baseband signal up in frequency so that it can be transmitted at for example 200 MHz.

In your diagram there are 2 mixers and an IF stage. This can be used to first mix up to 10 MHz (right mixer), then filter that signal, for example only let frequencies between 9 and 11 MHz pass, attenuate the rest.

Why then not mix directly to 200 MHz and filter there, saving a mixer ?

Well, a 2 MHz wide filter at 10 MHz has a Q factor of 10/2 = 5 which is easy to make.

At 200 MHz that would be 200/2 = 100 and that is much harder to make, more sensitive to drift, proper tuning etc. It would need to be better than 1% accurate. All in all not so easy and for sure expensive.

Then in the left mixer mix this 10 MHz IF (and modulated) signal with 190 MHz (or 210 MHz) resulting in 200 MHz for the RF amplifier.