Keeping your items clean: We've established that you don't want to have your sides covered in crockery because they would look highly cluttered. However the other reason you don't want cluttered sides is that it would also mean that the items themselves were exposed to the dust and dirt and would likely become rapidly unclean. In turn this would result in them being too unhealthy to use. At the same time leaving your items out on the side would mean that flies landed on them, and that they were maybe exposed to dogs jumping up and licking them etc... all of which demonstrates how important kitchen cabinets are.
Keeping young children safe: Leaving knives out on the side would be dangerous for any of us. However for children this would be even more the case and they could easily grab them off the side and injure themselves.