Why might government require that groups first obtain permits to parade or demonstrate

Governments might require that groups first obtain permits to parade or demonstrate because those events might block traffic or require detours for people trying to get to work, school, or to run errands. Permits are usually required because while most governments allow certain freedoms, that doesn't mean every citizen wants to partake in those freedoms all the time. As such, the government has to make sure that the different groups all feel comfortable with the situations.