Each speaker is designed for a specific frequency response.

Tweeters want high frequencies and will distort if bass or low frequencies are sent to them. Some midrange frequencies can distort them badly too.

Midrange speakers as the name suggest are designed to handle "middle" frequencies. Usually above 200 hz and below 10,000 hz. Above 10 kHz is where the tweeters hang out but this doesn't distort the midrange speakers much as they just can reproduce it.

Low range or bass speakers or subwoofers are designed for very low frequencies. Midrange signals while not distorting them shows up as distortion since they cant move fast enough to really respond cleanly to them. High frequencies are just ignored completely by subs.

So each of the 3 types need to be fed a signal they can work with. THATS what the crossovers are for. The factory amp has these built in per channel. Rare actually.

Aftermarket amps like the one your looking at have crossovers but not enough channels to drive all the speakers you have. So some (if you use one amp) are going to overlap. That's where external crossovers come in. To use on the speakers in place of the inputs to the amp or the amp itself.

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Aftermarket E53 Radio Install