xombie: Don't forget the Black Knight, which is also quite useless.
Black has a terrible White square weakness in the end. Look at the agonizing cat and mouse game moves 44~ after the rooks have been exchanged. Petrosian threatens to invade on h7, and is thwarted. The white Queen finally settles on the a4, e8 diagonal, whence penetration occurs on e7, e8 (if Qc7)or b7 (if Qd8)-all on the white squares.
Incidentally, Black seems to have no time at all-he arduously gets the knight out only to find that the white square weaknesses are too serious.
We may extend Thunderstorm's comment to all of Black's pieces, including the Queen which is too overworked to do everything at once.
One may also notice how Petrosian skilfully exchanges off all of Black's active pieces
a)21. Nb6 gets rid of the good White squared Bishop
b) The opening of the Rook file to exchange one pair of rooks
c) The same with the other pair
d) One of the knights-though I think Black exchanged this pair because he couldn't stand White's Knight on b6.
There is some masterful prophylaxis (or is it?) with g4. If fxg4, we have fxg3 and white's position is too 'distant' to be attacked-his bishops do a great job watching over them too. Black's threats are neutralized.
But I find 23. Kh1 is intriguing. Perhaps Tigran wanted to get away from the diagonal and file. Even so I can't see what he saw there.