The move order matters, for detecting threefold or fivefold repetition, or the 50 move draw limit. To adhere to that properly, you need to somehow represent al

The move order matters, for detecting threefold or fivefold repetition, or the 50 move draw limit. To adhere to that properly, you need to somehow represent all previous positions that could be reached again, and the number of times it has occurred. Of course you can get that by including all the move history, but it's also possible to prune it a lot, like any capture or pawn move can flush the history since no previous position is reachable. But it's still a bit more complicated than just representing the current position. FEN doesn't account for this, deliberately leaving the history out of scope. It's a matter of preference whether you'd want a tool like this to handle those cases.