ORIGINAL: siclick33
Page 21 of the manual, under 'Nosegear'.
You can also use it as a stop for the nosegear door. You can just about see it on the photo at the very top of page 21 (above the photos showing the U shaped guide). The strip can be glued to the inside of the fuselage, sticking out just a little, and sanded so that it only sticks out a couple of mm. This just stops the nose door closing too far.
As Darren says, it stops the nose door going inside the fuselage when it closes. I leave approx 3/32" showing (depends on your nose door clearance)
I face each side of the nose balsa block and hollow it slightly to help the block stay centralised running up the leg as it retracts.
There are often quite a few added bits included in CARF kits as they realise people have various ways of achieving the same thing.
Dave