I love how this ^^^^ is his "I don't have time to get in depth" answer. Lol.nice thread.
i don't have time right now to go into too much depth, but the overall message is that you are interested in the size of the sound sphere at the listening position (knowing that it's not a full sphere due to reflections and boundary surfaces). while you can delay the time the signal leaves the speaker, you cannot change the size of the sphere at the listening position. this is less crucial in a car but a huge deal in a large auditorium or performance hall, and is most applicable when designing rear monitors (like under a balcony).
time alignment will change the way the two spheres interact, and thus change the lobe shape and orientation. with time alignment you can "steer" the lobe. think about what happens as one speaker's sphere is allowed to grow larger before the other speaker starts.
comb filtering got its name because the response graph (dB vs frequency) looks like the fingers of a comb due to phase interference (both constructive and destructive). when two sources arrive at different times you will have both cancellation (at frequencies who's path length differs by 1/2 wavelength) and combining (at frequencies who's path length differs by a full wavelength). you also get that at harmonics of those frequencies - hence the comb shape in the response.
all of this is simple in free space with no reflecting surfaces. each reflection is another source. as sound continues to play, it keeps reflecting and you have more and more sources. increase room absorption and diffusion of sound, and you can stay focused on what is playing now, not what was playing a few seconds ago.
in a car - the door is the main culprit for acoustic distortion. sound inside the door cavity keeps bouncing around, building, until it eventually comes back out through the speaker cone as a muddy mix of crap. this is why songs seem to deteriorate the longer and louder you play them. doors with adequate absorption inside them (by adequate i mean as much as possible) do not suffer the same fate. the same concept is true for the cabin or listening space. lower reverberation time and you improve accuracy of the listening experience.