The aim of Employment law is to regulate the voluntary exchange made between employer and employee. Voluntary? Why should a voluntary exchange require any form of regulation? There are two main reasons.
First because the employment relationship is socially important and wherever you find a socially important relationship, there you find The Law. That’s what law is for – to identify socially important situations and jump right in with a whole load of salutary and worthy opinions about the Moral Ideal that people should be striving for. You can’t just leave people up to their own devices because you’d end up with a society that’s lawless and chaotic. Before long, greedy industrialists would be sending women and children into the factories and we can’t have that. It would be the Dark Ages all over again. Intolerable.
Second, because someone in authority has to keep tabs on all the unfairness going on in the workplace. That’s what the government is for. Without the government there to remind them, employers would soon forget that the workers are actually human beings. They would begin to treat their workers like robots – expecting people to work all hours without a break, overlooking the fact that unlike machines real people have interests outside of work, etc. [Editor: of course, very soon there actually will be robots available to do most jobs, which will create a very awkward situation for the law].
In sum, there are two main goals in this area of law: to push everyone towards the Moral Ideal that they should be striving towards (justice, fairness, sharing out all the wealth equally, etc) and to distinguish between human beings and robots (since employers have considerable trouble working this out on their own).