FWIU, It removes that body 'line' so you have a smooth surface instead of a nasty welded lip. Along the same lines of de-badging and rubstrip removal, it's something different from the others. Rain tray is kinda old school too, and not really necessary and if you go custom, it's that extra little easy thing to do.
Raintrayectomy is also a sign of someone who is adept in MKI.
