what are splits used for and why are they instaled over 6x9's or 6"

I believe they sound alot "clearer" and are more versatile with a separate tweeter that can be mounted in a variety of ways. Thats just my opinion though.
I use components (splits) in both the front and rear.

that is correct jastel, holden have factory 6x9 cutouts and 6x9's sound shocking in anyones language, unless you dont want a sub then they are a mild fix. you would use a split over a co-axial to gain a higher frequency response from your setup by moving the HF cone away from the midbass driver to avoid crossover distortion and there by gaining a higher sql (sound quality level)

how ever coaxial 6's are excellent for rear fill due to their simplicity