Clow in XXXHOLiC

The first time Clow Read is mentioned in XXXHOLiC, is when the main character, Kimihiro Watanuki, finds a fake staff that looks exactly like Sakura's in his employer's attic. Yuuko -who's the lady that "hired" Watanuki as a servant in exchange of granting his wish- is a powerful wish known between dimensions. She tells Watanuki that staff was created by a magician years ago, and that she knew him well as they're in the same business.

Later on, we know Clow and Yuuko worked and traveled through different worlds together, and one of the things they made were two clones of Cephiro's God, Mokona -from Magic Knight Rayearth-, and trained them so Yuuko could give one of them to four travelers from other worlds that crashed in her yard. Of course, it's obvious it's all part of Clow's plan, as two of these travelers are alter versions of Sakura and Syaoran (see the Tsubasa section for more on them.)

The whole role of Clow in the XXXHOLiC plot is still uncertain, but we do know he knew about Watanuki beforehand. What does he have to do with anything?