Heel Cap
Just for decor
Most instrument necks have a heel cap which covers the flat end of the heel. There's no structural need for a heel cap, but the neck would look less complete without something to "finish off" that portion of the neck.
Here are a few guitar heel caps:
An ebony cap that matches the binding:
An ivoroid heel cap that matches both the binding and purfling:
The lines under the heel cap are laminations, not purfling. It's much easier to simply stack up a few layers on a small surface than it is to inlay the lines around tight corners!
An ebony heel cap that contrasts with the binding and matches the purfling: