Huh, I remember actually being taught this at school, but they never bothered to give (or I never bothered to remember) an example of a programming language that actually named void functions differently or indeed why it couldn't just be a void function. Looking at it now, it seems to be a difference inherited from mathematics, which would also explain why it's in Fortran too.

The main difference between functions and subroutines in Fortran and other ancient programming languages is not the fact that subroutines do not have a return value.

The functions of Fortran are what would be called pure functions in C (which can be marked as such with compilers that support C language extensions, like gcc).

The pure functions cannot modify any of their arguments or any global variable, and they must be idempotent, which is important in program optimization.