There's a psychological component to it too. If it's possible to calculate forward in the variation without too much trouble, taking the time to think right away forces your opponent to visualize further too - and gives you the opportunity to fire off several moves rapidly afterwards since you've already calculated it through.
That makes it more likely your opponent misses some trap, even if he didn't get drawn into your tempo he would have had lacked the option to think or (in longer time controls) stretch his legs on your time. There's nothing that makes people think "trap" as much as a long time followed by leaving something hanging. 3 fast (precalculated) moves and maybe a facial expression saying "I just realized I blundered" makes the same trap much more likely to succeed.
Reminds me of the article where Najdorf was playing Gligoric at the 1952 Olympiads:
After Najdorf played 39. Nxd3, he facepalmed, pretending to have overlooked 39...Nxe4, after which 40. fxe4 Qxe4 wins back the knight.
Unfortunately for Gligoric, who had fallen to Najdorf’s trick, he unassumingly played 39...Nxe4, after which Najdorf instead played 40. Qe3, winning the knight.
Dirty trick, yes, but also darned effective.
There's a psychological component to it too. If it's possible to calculate forward in the variation without too much trouble, taking the time to think right away forces your opponent to visualize further too - and gives you the opportunity to fire off several moves rapidly afterwards since you've already calculated it through.
That makes it more likely your opponent misses some trap, even if he didn't get drawn into your tempo he would have had lacked the option to think or (in longer time controls) stretch his legs on your time. There's nothing that makes people think "trap" as much as a long time followed by leaving something hanging. 3 fast (precalculated) moves and maybe a facial expression saying "I just realized I blundered" makes the same trap much more likely to succeed.
True art of war