The Borg want civilizations to evolve to counter them. They thrive on "anti-Borg" technology being developed and instituted. Their policy, at least in regards to the bigger polities like the Federation and probably the Romulans, is to send out sacrificial cubes to push them to develop these "out-of-the-box" techniques and countermeasures, which they are incapable of conceiving on their own.
That way, when they inevitably conquer their foe, they are smarter, stronger, and closer to perfection.
Pretty much what I was getting at, that and the idea that by stimulating technological development they are making the target civilisation all that more attractive for assimilation down the line.
It's basically turning the perceived implications of the collective's size upside down. We go from "they have thousands of cubes so why not use them en masse to save attritional losses" (conventional military thinking) to "they have thousands of cubes so those attritional losses are worthwhile if they result in a greater payoff down the line" (assimilation thinking - they see those civilisations almost as crops to tend, attacking them in the hope they will become tastier, manufacturing and distributing tech en masse which the Borg can consume)