This question tries to illuminate why it is probably good if within a country/an economy there are more jobs rather than fewer jobs. I would agree, it seems to me that employment is likely to lower the chances of poverty and crime, and modern western societies have placed a lot of value on being employed (or working in general, as in being self-employed or even owning and running a business - but most people still work regular old jobs). Thus, as a society, we probably should be interested in as many people being employed as possible.
That being said, why do we levy taxes on income from labor, as most if not all countries seem to do?
I understand there are practical reasons to do so: It is relatively easy to do. It has been done for a long time and is widely accepted. It is not easy to get rid of the tax, because of its large share in government income. But I am interested in the justification, not the practical reason.
I am also (currently) not interested in the tax rate for income from labor compared to the one for income from investment, such as owning shares in a company or renting out housing. There are many great arguments to be had over them, but that shall not be the purpose of this question.
One possible answer is that the government provides a framework that must be paid for but allows an individual to work: enforcement of contracts, public safety, national defense etc. But this in itself is an argument for taxes, not an income tax on labor.
For the purpose of this question, I would define labor as working for someone else, who is not a customer. Examples would be: Alice works in a retail store under a contract that is not limited in length. Bob works in a factory through a temp agency. Nonexamples would be: Eve works as a consultant, is hired for limited amounts of time by companies and renegotiates the terms every time. Kim is a lawyer, working in a one-person firm and looking to expand, not join another firm.
Further, I define taxes as payments that go into the general funds of the government, i.e. not the contribution to mandatory health care insurance, mandatory retirement insurance, mandatory accidental injury insurance etc. Some people see those as taxes - I do not, I consider them premiums.