Governments may choose to intervene in markets for various reasons. These reasons usually result in an overall goal of changing the allocation of resources to what is viewed by politicians and their constituents as an improvement in the distribution.
The main and most logical reason for intervention is to correct market failures (situations in which equilibrium levels produced by the free market are inefficient), especially in the case of externalities. In the case of externalities, the market is not accounting for social costs and social benefits that affect third parties not involved in the market transaction. Governments may intervene to correct for this using taxes to help account for social costs and subsidies (money given by the government to businesses to keep prices low) to help account for social benefits. Other reasons include trying to achieve a more balanced distribution of income, or engage in price controls, taxes, or subsidies through legislation. These types of intervention usually have a goal of benefiting constituents or shifting allocations to a more favorable outcome, but often come with the tradeoff of a social or deadweight loss, which is loss to the economy caused by resources being used inefficiently.