user2991: I am reading Kasparov Teaches Chess, and I had a problem about this situation below:
[FEN "R2K1B1R/PPP5/5P1P/3PP3/3N1pB1/3p1np1/pppb1qbQ/r2k1r2 b - - 0 1"]

Black plays Ra8? next, why? I think it's better not to leave the queen unprotected.
Sorry I may not have made myself clear. Kasparov gives this variation to demonstrate a certain response is a bad move. I know Ra8 is bad. But if there aren't good choices, you can't say this variation is actually bad, isn't it?
This is the original board situation:
[FEN "rn1qk2r/pb2bppp/1p2p3/1Bp5/3PP3/P1P2N2/2Q2PPP/R1B1K2R b KQkq - 0 1"]

1... Nc6 2.Ne5 Rc8 3.Qa4 Qc7 4.Qxa7 Ra8? 5.Bxc6+{White Wins}

And by giving such a variation Kasparov made a conclusion that Nc6 is a bad move. What if Black don't play Ra8? He can still possibly make equal trades, and thus Nc6 isn't really bad?
The point is, why is Nc6 a bad move?

user2042: 
Why Black play Ra8 in this situation?

He wanted to trap the White queen.

I think it's better not to leave the queen unprotected.

You are correct, Black made a blunder, and now loses the game: 
[StartFlipped "0"]
[fen "2r1k2r/Qbq1bppp/1pn1p3/1Bp1N3/3PP3/P1P5/5PPP/R1B1K2R b - - 0 1"]

1...Ra8 2.Bxc6++- Qxc6 ( 2...Bxc6 3.Qxc7+-) 3.Nxc6! Rxa7 ( 3...Bxc6 4.Qxb6+- ) 4.Nxa7+-

Your question is not entirely clear but I hope this post answers it. If you need help or you have further questions leave a comment.
EDITED IN RESPONSE TO THE OP EDIT:

The point is why is Nc6 a bad move?

Nc6 is a bad move because it allows a decisive pin along the a4-e8 diagonal. The fact is that Bb7 is badly posted to defend the knight on c6. If he were at d7 then Nc6 would not be pined and Black would be fine.
After Nc6 White is able to exploit the bad positioning of the Bb7 to net himself a pawn on a7, and still keep the pressure on Nc6. After White captures on a7 we have the second pin on Qc7 which paralyses Black. Now he must castle, but then White can win a piece:
[StartFlipped "0"]
[fen "2r1k2r/Qbq1bppp/1pn1p3/1Bp1N3/3PP3/P1P5/5PPP/R1B1K2R b - - 0 1"]

1...O-O 2.Bxc6 Bxc6 3.Nxc6! Qxc6 4.Qxe7 $18 Qxe4+ ( 4...cxd4 5.O-O ) 5.Be3 Qxg2 ( 5...cxd4 6.cxd4 Qxg2 ( 6...Qd3 7.Rd1 $18 ) 7.Kd2 $18 ) 6.O-O-O $18

I have checked the lines with the engine. If you have any doubts you can run these positions through the engine yourself. The point is that center is fixed, and White can defend himself by exchanging rooks on the c-file. Furthermore, bishop defends well too, and queen can come fast to defense. Black may be able to make a pressure but it will not be a permanent one. White will untangle eventually and convert his extra piece.
If you still have doubts you can post the position as a new question and we can see if other members can spot something that I did not.
END OF EDIT
Hopefully this helped.
Best regards.

2991: <p>I am reading <code>Kasparov Teaches Chess</code>, and I had a problem about this situation below:</p>

<pre><code>[FEN "R2K1B1R/PPP5/5P1P/3PP3/3N1pB1/3p1np1/pppb1qbQ/r2k1r2 b - - 0 1"]
</code></pre>

<p>Black plays Ra8? next, why? I think it's better not to leave the queen unprotected.</p>

<p><strong>Sorry I may not have made myself clear. Kasparov gives this variation to demonstrate a certain response is a bad move. I know Ra8 is bad. But if there aren't good choices, you can't say this variation is actually bad, isn't it?</strong></p>

<p>This is the original board situation:</p>

<pre><code>[FEN "rn1qk2r/pb2bppp/1p2p3/1Bp5/3PP3/P1P2N2/2Q2PPP/R1B1K2R b KQkq - 0 1"]

1... Nc6 2.Ne5 Rc8 3.Qa4 Qc7 4.Qxa7 Ra8? 5.Bxc6+{White Wins}
</code></pre>

<p>And by giving such a variation Kasparov made a conclusion that Nc6 is a bad move. What if Black don't play Ra8? He can still possibly make equal trades, and thus Nc6 isn't really bad?</p>

<p><strong>The point is, why is Nc6 a bad move?</strong></p>
 2042: <blockquote>
  <p>Why Black play Ra8 in this situation?</p>
</blockquote>

<p>He wanted to trap the White queen.</p>

<blockquote>
  <p>I think it's better not to leave the queen unprotected.</p>
</blockquote>

<p>You are correct, Black made a blunder, and now loses the game: </p>

<pre><code>[StartFlipped "0"]
[fen "2r1k2r/Qbq1bppp/1pn1p3/1Bp1N3/3PP3/P1P5/5PPP/R1B1K2R b - - 0 1"]

1...Ra8 2.Bxc6++- Qxc6 ( 2...Bxc6 3.Qxc7+-) 3.Nxc6! Rxa7 ( 3...Bxc6 4.Qxb6+- ) 4.Nxa7+-
</code></pre>

<p>Your question is not entirely clear but I hope this post answers it. If you need help or you have further questions leave a comment.</p>

<h2>EDITED IN RESPONSE TO THE OP EDIT:</h2>

<blockquote>
  <p>The point is why is Nc6 a bad move?</p>
</blockquote>

<p><code>Nc6</code> is a bad move because it allows <strong>a decisive pin</strong> along the <code>a4-e8</code> diagonal. The fact is that <code>Bb7</code> is badly posted to defend the knight on <code>c6</code>. If he were at <code>d7</code> then <code>Nc6</code> would not be pined and Black would be fine.</p>

<p>After <code>Nc6</code> White is able to exploit the bad positioning of the <code>Bb7</code> to net himself a pawn on <code>a7</code>, <strong>and still keep the pressure on <code>Nc6</code>.</strong> After White captures on <code>a7</code> we have <strong>the second pin on <code>Qc7</code></strong> which paralyses Black. Now he must castle, but then White can win a piece:</p>

<pre><code>[StartFlipped "0"]
[fen "2r1k2r/Qbq1bppp/1pn1p3/1Bp1N3/3PP3/P1P5/5PPP/R1B1K2R b - - 0 1"]

1...O-O 2.Bxc6 Bxc6 3.Nxc6! Qxc6 4.Qxe7 $18 Qxe4+ ( 4...cxd4 5.O-O ) 5.Be3 Qxg2 ( 5...cxd4 6.cxd4 Qxg2 ( 6...Qd3 7.Rd1 $18 ) 7.Kd2 $18 ) 6.O-O-O $18
</code></pre>

<p>I have checked the lines with the engine. If you have any doubts you can run these positions through the engine yourself. The point is that center is fixed, and White can defend himself by exchanging rooks on the <code>c-file</code>. Furthermore, bishop defends well too, and queen can come fast to defense. Black may be able to make a pressure <strong>but it will not be a permanent one.</strong> White will untangle eventually and convert his extra piece.</p>

<p>If you still have doubts you can post the position as a new question and we can see if other members can spot something that I did not.</p>

<h2>END OF EDIT</h2>

<p>Hopefully this helped.</p>

<p>Best regards.</p>
