KDE 3.5.x had an option for enabling composite called kcompmgr (which was a modified version of xcompmgr), it had basic functionality like transparency and fading, but nothing else, and it wasn't exactly fast.
Then Compiz came, originally designed by Novell along with XGL, but, even if it can be used with KDE, it obviously was designed with GNOME in mind.
The KDE developers wanted eye-candy composite for they KDE 4 series, so they had 3 options, writing a whole new window manager, use compiz or improving kwin, they took the last one.
That way KDE didn't lose any advance window management feature and won 3D window management full of eye candy and some useful features
Lets star by one of the useful plugins, originally designed by apple, it has 3 pesent modes:
Natural, it tries to preserve the size relationship between the different windows: