Why do you calculate formal charge?
So teachers will have something tricky to ask students; what other reason could there be?
Most of the time, calculation of formal charge is pretty straightforward. We usually start with neutral compounds, and thus we will finish with neutral charges even if the charges separate on reaction. See this site for examples. Remember what I said about the conservation of charge.
In redox reactions, assignment of formal charge is crucial, as it is required to balance the equation.