These files are related to 3d models and 3d animations. Let's say:
file named "FireLance01.efx" is related to fire lance, and "AquaLance.efx" is related to aqua lances. You can either replace those two files, so fire lance looks like aqua one, or you could completely remove them, so if you cast your lance, no animation will have been shown.
Might cause crashes.