The control structs are the statements the language offers you to have the control over your application. They are used for mantain a more logical behavior inside the program.
There are differente kind of them: conditional ( they decide if an action or other must be taken ); repetitive ( certain group of actions to be taken n times ); and so on.
Without control structs, any application would be so boring and so useless. Understanding the control structs is a fundamental part of the learning of any programming language.