Sure! Printers were slow and paper not an unlimited resource, so you can hide stuff that you don't need printed. For example, commonly-used macro definitions,

Sure! Printers were slow and paper not an unlimited resource, so you can hide stuff that you don't need printed. For example, commonly-used macro definitions, SINE tables, or hardware drivers. It was just a tool to print only the stuff you were debugging at the time. Of course it can be abused if the only thing being delivered is a hard copy. I'm a bit surprised that the original source code wasn't delivered and the customer built it themselves. But assemblers often ran on mainframes, and that wasn't cheap.