It allows you fill the tank right to the top very easily - something that is not possible as standard.
I did this mod to my GS but to be honest I did not bother taking all the panels off and lifting the filler neck - I just carefully drilled it whilst in place. I did place a rag down the nect to stop the swarf going into the tank.
I did this mod on my R1150 after seeing it on a merrycan forum...
not necessarily to enable morer litres in, but allows faster/easier fills cos' the air comes out of the holes and helps control the 'blowback' syndrome.