They really wouldn't be needed if they already had a crossover built into their amp or headunit. They might add one anyway because they feel the headunit or amp isn't doing a good enough job.
a- the EQTs are 30 band equalizers for shaping the response of the sound curve. They're used to compensate for the cabin environment since cars are notoriously bad settings for good audio response.
b- the 300PSW isn't an EQ or XO, it's a pre-amp. I'm actually not even using that pre-amp at present as with the 7990 Alpine I don't really need it.
c- the 200CRX is an electronic crossover that allows me to control both high and low pass frequencies for my subs and front stage, and the Orion 2250SX amplifiers came out around 1990, long before an EQ and XO were standard features on an amplifier. This was prior to SMT boards, when everything was still solid state components, so things took more space inside the amps.
d- the 400BDG is a bridging module, which allows me to use the DIN level outputs of the 200CRX and have one pair of channels as an input to the bridging module, and for outputs I have 4 pair of channels.. left, right, mixed mono and summed mono. in my case I use left and right DIN outs to the amps for signal splitting. The Orion components allow DIN cables with a balanced signal path and 15 volt line driver so I don't have any noise in the system.