the-witness
I found and solve a puzzle in the Marsh for which i don't find any use
This puzzle is under the water, after the 3 square moving platform.
Here is a picture of the location : 
And here is the puzzle : 
Can anyone tell me the purpose of this puzzle ?
Both solutions are correct for this puzzle.
However, this puzzle controls the movement of the platform you're standing on. The first solution is for the side you were already on.
If you need to go back across to the other side, you will need to use the first solution.
There's a possible interaction between your numbers 1 and 2 that you're missing. The blue blocks only subtract from yellow blocks that are in the same space. Let's work with an example 2 box puzzle. If there's one blue block in one box, and a yellow block in the other, the solution is to go around the edge of the puzzle, not to go through the middle, splitting them up. If you cordon off the blue block from the yellow block, then the blue block doesn't do its thing, so it's not a valid puzzle solution.
So, be sure that you design a solution that has all the blue blocks in the same space as yellow blocks.
And yes, this is a super tough puzzle, it took me quite a bit of fiddling to get everything in place.
As you requested in the comments, here's a quick solution. Took me less time this time around, since I vaguely recalled some of the tricks I used in the solution.
Best Answer
Without getting in a discussion about purpose of playing the game in the first place... Well, you have +1 solved puzzle for every solved puzzle!
As far as my experience goes, a lot of end-game puzzles are only there to be there, as an additional challenge. This puzzle including: I have not seen any events after solving it.
Even more: AFAIK the game has very direct feedback when something happens: line turns on, there is sound (with a pause after sound for completing the puzzle), etc. Nothing here, so this puzzle is a dead end.