Because in the good old days you expected to share an expensive page printer with many, and it made sense to put a powerful embedded computer (for the time) to

Because in the good old days you expected to share an expensive page printer with many, and it made sense to put a powerful embedded computer (for the time) to do rasterizing within it. It could save on bandwidth, RAM, and storage. The RAM requirements for rasterizing a page would be quite costly at the time.