If they're calling a brain their original processor, these robots could be a result of successful transhumanism rather than conventional robots, ie, they could be humans that have made themselves into robots, rather than robots built for some specific purpose. In that case, they might create child robots and grow them over time to reproduce, just because they find that mode of reproduction familiar and wish to continue it.
I like to imagine that the childbots are made in the image of the parentbots, and the childbots eventually are tasked with picking out preferred larger parts as a growing rite.