Technically, there's no purpose in any of it. Playing the game and not playing it are the same.

It's a puzzle in that it's a line you have to draw to complete all the puzzles. What's that worth to anyone is what it is. There are lots of games with post-game content that's not technically required to "finish" the game.

But the goal of the Witness is to find and draw these lines. If you didn't find and draw that line, Blow would say you didn't find and draw all the lines.

So, it is a puzzle in the game. It is. It's a puzzle. It's in the game. You don't know whether it's pointless or not until after you complete it. So you can't look at it and say, "Well, this is not needed, I can skip it".

And the overall product is kind of bad. It's overly tedious and mistakes hiding necessary information with being clever.