There isn't a practical reason for the shape.  It looks good and is decorative. 
Medieval nocks were less elaborate, more of an elongated bullet tip. 
Many horn tips naturally have a curve that suggests a horses head and neck. This gets accentuated by various bowyers over the centuries.  
Instead of the abstracted suggestion, realistic horses heads like a chess knight, griffins, or dragons have been known to be carved onto the top nock.