The Pattern Notcher is used to clip the edges of the block where something needs to occur in the construction of the garment; e.g. at the end of the dart legs, so the sewer knows how much of the fabric to fold together to construct the dart; balance points, so the sewer can match two garment pieces together, etc. The marks made by the notcher are transferred from the block to the pattern and fabric to ensure the garment is constructed correctly.
Notching the Bodice Block Front
In this image, the armhole and the dart legs are notched on the Front Bodice. The armhole has one notch to show that this is the front armhole (the back armhole will have two notches) and it is also a balance point to assist attaching the bodice to the sleeve; the notches on the dart legs are to show how much fabric is to be pinched together to form the armhole and waist darts.
Notching: Marking the Inside of the Pattern
The hole that is clipped in cardboard allows the mark(s) to be made in the inside of the pattern as it is being created, as in the example above.
If the marks are made on the outside it is possible that they won't be transferred to the inside of the pattern , and when the pattern is cut this important construction information will be lost.
Note: Related Tool
For the tool used to mark points on the inside of the block or pattern, see the Awl.