What is the purpose of a masters in public policy?

The purpose of Public policy is to make policies or schemes for welfare of public(the people). Policies are documents which outline what type of benefits govt will provide to people or what laws should the people follow.

There are many type of policies:
welfare policies(food stamps, unemployment benefits etc) ,
Internal Security policies( how to deal with terrorism or law and order situation),
Agriculture policies (Agricultural subsidies, No taxation on Agriculutre etc),
Corporate Policies (Tax benefits, Tax laws, security laws, transaction laws, banking laws, etc)
Transport policies (Motor Vehicle laws, Road taxes, Insurance etc)
Military policies
Foreign policies ( how to deal with certain nations, These nations can be friends, foes or neutral)
Environmental policies ( saving forests, National Parks policy, Ozone depletion policies, Global warming policies, Greenhouse Gases policy etc)

Public policy will teach you what values, ideals, etc are required to deal with each kind of policy. If you take welfare policies, the values can be equality, right to food, right to employment, etc. You will study about many thinkers who have given different views on these values.

After completing the course, you can join a think tank which will help the govt make the policies or you can join the govt which will make the policies or you can join an NGO/pressure groups or corporate lobbies which will nudge the govt to make a policy favourable to certain groups( this group can be corporate, LGBTQ, environmentalists etc)

Hope This helps