The religion since old times has worked as a prominent way to dominate and conquer other territories and their people, in Charlemagne times (742-814) the catholicism was already really popular, his father was a devotee and had great relationships with the pope and Rome. he followed the same behavior and used the catholicism to start to culturize and unite the conquered territories changing their beliefs.
Besides the political and cultural reasons, Charlemagne was a strong believer, that genuinely thought about the benefits of a Cristian Empire, where the Emperor and pope work to each other, with all the benefits that entail, besides placing God in the process.
So the monasteries worked as saint places, used to teach and expand the Christianism to different territories across Europe, a lot of people were illiterate in the era, so besides work as a guide for their religious beliefs, it helped to educate and teach common knowledge. This pleased Charlemagne and considered a benefit and a need to designate places for this kind of purpose.
