I mean hasn’t it supported the interests of both to some degree, but only when the working class forced it. As a rule, it tends to support business because that is where wealth is concentrated and influence follows. The most substantial gains in worker rights has always been through unions, strikes, civil disobedience, etc. acting as a catalyst for politicians to prioritize the interests of workers over business. In other words, workers need unions to be more powerful than business.
Where are you trying to go though with your questions? I still think private interest serves a role in organizing society for certain important tasks by providing an incentive structure that the government can’t. That said, the government should heavily regulate private interest so it mostly serves the best interest of the public.