Most likely you did not hire your staff just because they have experience dealing with cleaning lunch rooms, offices, restrooms etc. Most of the time there will be a lack of quality in the cleaning due to not being experienced and not enough care being put into it because they may have other important things to deal with.
With a commercial cleaning company, their sole job is to clean and sanitize your building. They are experienced and have done it for a while. Being aware of all the germs in and around an office, building is extremely important as well as the chemicals needed to eliminate them. Not being subjected to constant industry training could create a drawback in the look and appearance of your space.
Dealing with certain aspects of commercial cleaning require knowledge and experence. Your staff may not know how to clean certain type of floor thus causing damages resulting in you or your office spending more than if you had hired a professional.