In my opinion, Sollozzo's kidnapping of Tom served no more practical purpose than to guarantee him an opportunity to make his pitch to Tom, in a controlled environment, at a time when Sollozzo believed his own life to be in danger from Sonny (which it was). That was really the source of urgency for Sollozzo: to reach a deal and attain a sense of equilibrium quickly - before Sonny had a chance to strike back.

I don't view the kidnapping as Sollozzo's attempt to intimidate Tom into anything, as his only intention was to reason with Tom - not to threaten him. If you'll recall, he even made an attempt to put Tom at ease, by telling him: "I know you're not in the muscle end of the family Tom, so I don't want you to be scared."

Of course, when Vito survived the assassination attempt, that threw a wrench into Sollozzo's entire plan. Especially when it became clear that he had no Plan B prepared in the event of such a contingency. That really was the beginning of the end for The Turk.