Why 'javax.servlet' could be 'ProvidedCompile' in Gradle war plugin?

In Gradle war plugin, we could set some dependencies as 'providedCompile' which will tell Gradle to include them only in compile time, not in the packaged .war file. How do we distinguish which dependencies should be as 'providedCompile' or 'compile'? What will happen, if we make every dependency compile or 'providedCompile'?