The purpose is the same as the Ultimaker Hobbed Bolt Release by bkubicek ( http://www.thingiverse.com/thing:14505 ).
However, I have a different kind of hobbed bolt, so his model didn't match my bolt. Also, some nuts didn't allow the holder to turn, so the knurled part is separated from the extruder and the external diameter is slightly lower to fit loosely with the extruder motor.
Print and insert the hobbed bolt head in the piece carefully. You can use a small hammer to insert the piece.
With this piece and http://www.thingiverse.com/thing:8903 you will be able to unmount the hobbed bolt very easily to clean the plastic inside before an important print.
Note: I printed it in standard quality in an Ultimaker. I used netfabb with a fill spacing of 1mm with the Standard fill profile in PLA.