Just as the title ask. Is there a particular function?
...barber could pull the razor out of a pocket and use the notch to catch the blade and make for a one handed opening.
Good for getting errant nose hair on the bigger blades.
I'm thinking it's there to allow easier maneuvering under the nose.
I thought it was obvious that this was the reason.
I have even seen Seraphim put one on the heel of the blade.I'm sure that it had a function when it was first added to a razor but as time went on, the barbers notch changed in size, shape and position so I think it ended up becoming more about aesthetics.