First off, why would you want to remove those filters ?
Well, they get in the way when you want to receive the uplink frequencies of the GSM band, attenuating the signal by around 30dB. The two main applications where it's useful to receive those are :
Using the phone as a passive listener ("sniffer")
Using the phone as a base station ("BTS")
This step is just the "preparation" of your phone hardware for those applications. A lot of further work is needed to actually use the phone like that (if it's even possible), but this is outside the scope of this document.
