Money, legal, strategy, and dealmaking. Thiel was trained as a lawyer, then worked for Credit Suisse and started his own hedge fund.

If you read the Max Levchin interview in Founders At Work, the way PayPal started was that Max wanted to do crypto (back when it meant cryptography, not cryptocurrency) at Stanford, attended a lecture by Thiel, and Thiel said "Go do it. My hedge fund will invest in you." Then there were about 4 pivots, then the company Levchin & Thiel founded (Confinity) merged with its primary competitor, Elon Musk's X.com, in a deal brokered by Thiel. Elon Musk's primary contribution was supposedly trying to rewrite the software in Windows (the interview was with Levchin, it's likely somewhat biased - Thiel is more complimentary towards Musk and says he was a fearsome competitor). Then it found product/market fit amongst E-bay sellers, rode the hyper-growth wave, went public, and was eventually bought by E-bay.