It’s because there is a limited number of board states + transpositions a person can reasonably memorize, and memorizing a board state + sequence of moves from

It’s because there is a limited number of board states + transpositions a person can reasonably memorize, and memorizing a board state + sequence of moves from a chess engine will beat human intuition. So the idea is to steer the gamestate into places your opponent did not prepare for but you did, with non-obvious moves. All near-optimal moves are obvious.