Why slope deflection method is used in structural analysis?

1.  Slope deflection method is a displacement method. In this method, unknowns are taken as joint displacement. 
2. Slope deflection method is used when kinematic indeterminacy (Dk) is less then static indeterminacy (Ds) that's why it is generally used for the analysis of statically indeterminate structure (Because when Ds>0, then the structure is indeterminate)
3. In slope deflection method all joints are considered rigid i.e. angles between members at the joints do not change with the application of load. 
4. In slope deflection method, to find unknown displacement joint equilibrium equations and shear equations are written.