I assume that you are referring to the idea that government is supposed to play the following six roles in the economy:
- Maintaining Legal and Social Framework
- Maintaining Competition
- Providing Public Goods and Services
- Redistributing Income
- Correcting for Externalities
- Stabilizing the Economy
If so, the only possible answer is the first function. When the government regulates working conditions, it is maintaining a legal framework that allows our economy to work properly. Without such a framework, workers could be abused and the system might fall apart as labor strife became excessive.
Some regulations have to do with correcting for externalities. An example of this would be regulating air pollution. But that is not the case for the regulation of working conditions since working conditions affect people within the company, not "external" people.