Here is an example
[FEN "8/3k2PB/q7/8/4K3/8/8/8 w - - 0 1"]
One might intuitively say g8=Q is a much better move than g8=R. However, since they both lead to a draw, should we not conclude that g8=Q is as good as g8=R?
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3Not paying attention to fact mentioned in answer(R - loss; Q- draw) You have to evaluate possible chances not only perfect play if the moves are equal with perfect game one of them still may give you more practical chances - putting Queen definitely gives you more chances than Rook typically.– DrakoAug 3, 2020 at 7:53
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2That really depends on your definition of 'objective'. In game theory, where we model both players as rational self-interested decision-makers, there is indeed no reason why we would choose to play one move over another if they led to the same result. 'Objective' can also be used to mean what is considered good by two strong (but not perfect) players. E.g. the Sicilian Defence is objectively better than the Scandinavian Defence, even though they both probably lead to a draw with perfect play. Remember though that chess is a game which is much deeper than 'making the right moves'.– JoeAug 3, 2020 at 18:50
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Isn't it safe to say that promoting to either Rook or Bishop (under any situation, not just this one) is at best equal to promoting to Queen, but generally worse? The only time you'd ever have a good reason to promote to anything other than Queen would be a Knight.– Darrel HoffmanAug 3, 2020 at 19:23
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@DarrelHoffman, There are situations where promoting to a Rook or Bishop is superior to promoting to a Queen, because promoting to a Queen would give a stalemate.– AkavallAug 3, 2020 at 20:09
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1Underpromotions to Rook and Bishop happened in actual games: chessgames.com/perl/chesscollection?cid=1004293– AkavallAug 4, 2020 at 4:45
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5 Answers
Set aside the particular example you use, and instead assume that we have found some state where two moves are strictly equal under optimal play. Why would you favor one over the other? Let's focus on endgames, since that's the simplest.
The answer, as pointed out by various comments, hinges on the fact that unless you're a masochist playing against a AlphaGo on max settings, you are unlikely to be engaged in optimal play, and thus the candidate moves are not equal in a practical sense. One position might be difficult, in the sense that there is only one drawing ply for you on each move that you have to successfully identify each time, while another is quite easy in that you have many options open to you on a given move. They are both equal under optimal play, but the latter is easier to manage.
Go play around with the Syzygy endgame tables and compare various drawn positions, and in particular the mistakes that are available to the player to move. It's likely that you'll have the sense that some are easy draws, and some are obtuse. For instance, KvK is a clear draw (no possible mistakes), while KQvKQP gives you many opportunities to make a mistake. KQNvKQNP is a tablebase draw in the arrangement, but note that if White plays the Nb5, then it converts to a winning endgame for Black. Seeing this is not at all obvious (at least to me), and other such mistakes are probably lurking. Similarly with different winning and losing endgames.
The strategy here is subtle, and depends in part on your own endgame knowledge and your assessment of your opponent's. Suppose you are playing White and are given the chance to trade into this KQNvKQBP endgame or some other endgame that is also theoretically lost. You know with perfect play it's a loss for White, but there are some opportunities for Black to mess up and for White to steal the win. Do you take the trade? If you're playing against a computer, no, since it has access to the endgame tables. Similarly if you know that your opponent has lots of experience with such positions. If instead you suspect you have more experience, then you might be willing to take the risk to capitalize on your comparative advantage.
Analogy with the opening
I thought that switching focus to the opening might help clarify the thinking here.
The two most common openings in chess are d2d4 and e2e4. I loaded up Stockfish 13 and had it evaluate each ply for a minute. The result is 23 CP for d2d4 and 22 CP for e2e4. Perhaps the former is objectively better, but for all practical purposes they are the same. Suppose you're in the final match of a tournament. Which one do you choose? Obviously, the one you know more about. If somehow you have managed to get to this point without ever playing d2d4, then you probably shouldn't start now, even though it is objectively 'better'.
Similarly, you might avoid going into an opening that your opponent knows more about. Think about how Beth wrestled with whether to play the Sicilian against Borgov in The Queen's Gambit. Maybe if you're a genius, you can get away with playing to your opponent's strengths, but I think most of us would avoid challenging people on their own terf.
Well, according to [Syzygy endgame tablebases](https://syzygy-tables.info/?fen=8/3k2PB/q7/8/4K3/8/8/8_w_-_-_0_1_, g8=R is a loss in 63 plies and g8=Q is a draw. So g8=Q is definitely better.
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You got me! Perhaps I should add another light-coloured bishop to white so after g8=R, the game is still a draw.– ZurielAug 3, 2020 at 14:45
In this case, g8=Q is certainly better, since it makes black have to struggle for the draw, and g8=R would make white struggle for the draw.
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Thank you! After g8=Q, black plays Qe6 and the game draws immediately.– ZurielAug 3, 2020 at 14:44
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1Ah, cool, still you have to find that, while as black in the position with Q vs R+B it's not totally clear to me that it IS a draw and as indeed, as showed by another answer, it isn't. Aug 3, 2020 at 14:51
Yes, if it prolongs and "shortens" the game if you're losing or winning respectively.
And there are tablebases for 7 pcs or less.
Which means chess is solved with 7 pcs on the board.
Anyway, if two moves certainly lead to a draw, then they are both evaluated as 0.00, so they are not objectively better.