For all of chess history, even the greatest players had been sheltered from the sort of incredibly complex tactical play that computers handled almost trivially by 1993. You knew that your human opponents had roughly the same limitations as you did when it came to dealing with whatever arose on the board.
[…] Generally, I always knew that if I couldn’t be completely sure of what the consequences of my move were going to be, my opponent couldn’t be sure either. That perceived equilibrium went out the window when you were facing a strong computer. It played chess well, but also differently.
The psychological asymmetry and physical factors […] were an issue, but the new sensation of always wondering if your opponent might be seeing something you could scarcely imagine was very disturbing. It created a terrible tension in complex positions, a sense of dread that at any moment a shot could ring out in the dark. In response, you double- and triple-checked your calculations instead of trusting your instincts the way you would against a human opponent.
—Garry Kasparov; source: Deep Thinking