The function of the outline layer is to show the board outline/milling/v-cut of your board, but some of user do not contain the outline layer in there gerber files and we don’t know how to cut the board. Like this:
How to deal with the problem:
Draw the board outline in an independent layer to show us the shape of the board clearly.
(Include the board outline, the millings, and the v-cut)
So that we can follow the board outlines to cut the board as below