A lot are subcontractors that work for the person you supposedly hire. If there are any problems it gives them deniability.
That was one of the first things I noticed when we built our house was it was NEVER anyones fault -- they always blamed someone else. Fortunately I know enough about wiring, plumbing, carpentry and the other trades and I was on site damn near every day that I was able to catch most of their friend-ups. A carpenter friend of mine told me I was the worst kind of person to build for. I just laughed. All I expected them to do was a good job.
We caught the foundation crew overlooking a bad measurement that would have left the house overhanging the foundation by about a foot on one end.
We caught the carpenters framing in some of the widow and door openings wrong so we would have had to re-order sizes.
We caught the plasterers starting to put a different texture on the ceiling and walls than what the wife wanted.
We caught our contractor "accidentially" charging some building supplies for another job to our project.
My wife and I both swore --never again.