A key element in structured programming is the use of modules. In Fortran, user-defined modules are implemented as functions or subroutines. Functions and subroutines are also called subprograms. Debugging is easier because you work with smaller sections of the program. Modules can be used in other programs without rewriting or retesting.
Programs are more readable and more easily understood because of the modular structure.
Several programmers can work on modules independent of each other. Individual parts of the program become shorter and therefore simpler. A module can be used several times by the same program. If an application requires a function that is not included in the intrinsic functions, Fortran allows us to write our own functions. The statement function is defined at the beginning of the program, preceding any executable statements.