Apple always wanted to get into ads business, but the contenders were too strong and Apple doesn't have an edge for competition unlike Google Search, Amazon Marketplace, Facebook's user tracking etc... So instead of directly jumping into the business they decided to first level the ground with IDFA and ITP. The natural next step would be to monopolize user tracking data within the Apple ecosystem. I'm almost sure that this is "Manifest destiny" for Apple, but the question is "when".