The default RepRapPro Mendel method of mounting the extruder with three tab clips is pretty bad; it's very floppy, the front clip can't be pulled up on properly, and it mounts the motor low enough that you can't use the full height of the printer.
This part replaces the 2mm thick spacer and sits with four U-shaped feet on to both top bars, resulting in the motor being about 10mm higher.