Knowledge of support reactions generated by a support is very helpful in analyzation of a structure. Support reactions generation knowledge is necessary to draw free body diagram of a structure . In calculation of indeterminacy also support reaction knowledge is very much helpful.
Support reactions are basically developed due to restraint of deformation provided by a support. In general any restrain on translation of a body provides a force support reaction in the opposite direction of translation. Similarly restrain on rotation of body provides a moment support reaction.