Why this move in tactic puzzle?

This is a weird one because there doesn't appear to be any clear tactic.  I get it that the knight needs to come forward for the attack, and maybe nothing can stop the queen and knight mating... But the engine says 2. ... dxe5  3. Nc5 b6  4. Kh1 bxc5  -- so what is the point of Nc5?! or at least, what's the point of not retreating the knight after 3. ... b6 ?   like why move it there in the first place, and then leave it to be captured... weird?

https://www.chess.com/tactics/143693

The puzzle is a simple hanging piece puzzle.

You are evidently looking at computer analysis for moves after the puzzle. That's not part of the puzzle.

If you are going to look at the computer analysis of moves after a puzzle ends, then the computer analysis will give you the best clues to understanding 

The above tells you that Stockfish thinks white has a better move than retreating the knight.

Still... not a very clear-cut puzzle.

It's not super clear cut, but I think the idea is that after we win a piece, our attack is over. He has completely neutralised our attack and the best thing we can do now is capture that one last pawn before we retreat and just try to slowly build up a solid position (at least that's my theory).