Thiel is a very smart guy. He appears to be rich and successful enough that he doesn’t need favors. He calls himself a “conservative libertarian” and has contributed to a lot of Republicans. Unlike other Silicon Valley billionaires, who appear to be mostly atheists, Thiel says that he is Christian. Maybe he admires Cruz as a fellow smart guy with legal training and conservative views. Thiel was interviewed for clerkships by two Supreme Court justices, but unlike Cruz, he was not accepted.
People don’t donate money unless they get a favor in return.