Same reason I made a child

Incredible foolishness

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.

Tight space requirements.

The childbots yearn for the tubes.

Almost as interesting as a robot who builds a museum.

Could just be a compact model, the big one could just be stupidly pointing out the obvious to a far more advanced compact model that's just given a lecture on the history of robotic processors.

The compact advanced model might just be about to say "Very good, I'm glad you were listening".

Maybe it’s a Grow-bot…

And why must the child-bot be so thicc?

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.

Small hands. Good for thievin', as the Ravagers say.

next iteration, the borg, biomech engineering