The function of the center channel speaker is two fold. One it allows the dialogue in a 5.1 system to be placed at a spot where it is coming from the same general space as the screen, so the voice sounds as if it is come right out of the mouths of the characters in the movie. In theory this can also be accomplished by sending the center channel signal equally to the front L & R speakers, but this assumes that those speakers are place in the room in such a way that you will get a good , clear center fill. In my experience, since most HT systems are in a shared living space, our wives usually insist that the speakers are placed more by where they will look good rather than where they will sound the best. Hence the center channel insures that the dialogue is placed properly in the sound field even when the L& R speakers are not in a optimal position.
The second advantage of a dedicated center channel speaker is that the center channel is used primarily for dialogue and as such has a much more limited bandwidth to deal with. A center channel speaker does not have to deal with the low bass and extended highs that a full range speaker does and as such the designer is free to optimize the speaker for only the midrange tones it will be call upon to reproduce. Of course this is not to say that all center channel speakers sound good. just as withe anything else there are some great sounding CC speakers and some really crappy ones out there. I have found that in my system , there is a definite advantage to using the CC instead of just the stereo L&R front speakers. But that is in my HT system. As with everything else in this hobby YMMV.
And just for the record, I am using a low end Sony 5.1 receiver and all Paradigm speakers in my HT System (A good Canadian company.
) If I was using better equipment perhaps I would not need the CC speaker, but this system throws a nice surround image in spite of the speakers being tucker against the wall and right next to the TV cabinet. I am sure it could be better, but for the amount of time that I spend in front of the TV, it is good enough for me.