system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Strange MCU] Why does Kaecilius need to steal pages from that book?

No knowledge is forbidden. Assumedly Kaecilius knew about the dark dimension before abandoning Kamar-Taj. Did he really need to steal those pages? Why not just quietly snap a picture with your cellphone before leaving for good/announcing your bad intentions? Or just study/memorize it (without practicing) before leaving and starting your own faction.
gpt
The point was to leave a message for the Ancient One, and not just to acquire the information. The book was deliberately tossed on the floor to be found, and the pages taken to so that the Ancient One would know that Kaecilius knew about Dormamu.