user26230: 
I don't understand how that move is a good move, much less the best move according to this chess.com analysis.

user13433: The picture already answers the question:
If you play any other move but ...Bh8, for example ...d6, White will answer g4 and trap your knight. Some exchanges can be thrown in before he can capture (dxe5, Qxd1 etc.), but ultimately, there's no escape.
...Bh8 by itself is indeed not a desirable move (moves a developed piece again, to a square with less influence), but here it's a tactical necessity to save that knight (which is more valuable than losing a move or two) by vacating an escape square.
Since White is behind in development, Black can actually afford to give that tempo without major damage (+0.31, says the computer).

user25320: The knight on h5 has no retreat squares, which means the threat of White playing g2-g4 is on the cards. The move ... Bh8 then frees up the g7 square so the knight has a retreat square should it be attacked.

26230: <p><a href="https://i.stack.imgur.com/wmwBY.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wmwBY.jpg" alt="enter image description here" /></a></p>
<p>I don't understand how that move is a good move, much less the best move according to this chess.com analysis.</p>
 13433: <p>The picture already answers the question:</p>
<p>If you play any other move but <strong>...Bh8</strong>, for example <strong>...d6</strong>, White will answer <strong>g4</strong> and trap your knight. Some exchanges can be thrown in before he can capture (<strong>dxe5</strong>, <strong>Qxd1</strong> etc.), but ultimately, there's no escape.</p>
<p><strong>...Bh8</strong> by itself is indeed not a desirable move (moves a developed piece again, to a square with less influence), but here it's a tactical necessity to save that knight (which is more valuable than losing a move or two) by vacating an escape square.</p>
<p>Since White is behind in development, Black can actually afford to give that tempo without major damage (+0.31, says the computer).</p>
 25320: <p>The knight on h5 has no retreat squares, which means the threat of White playing g2-g4 is on the cards. The move <strong>... Bh8</strong> then frees up the g7 square so the knight has a retreat square should it be attacked.</p>
