In reply to MayurKubavat:
I think we have come full circle in this thread. There are two different but related issues here.
The reason we split code up into multiple files is for readability and maintenance. We don’t want multiple people modifying the same file at the same time. It may be hard to appreciate this if you’ve never worked on a project with multiple people trying to modify the same code, which is why I was asking the question.
Once the code is in multiple files, we use different file extensions to help us remember if the file needs to be put on the compiler’s command line or will be included from the source of another file. We use the 'h' suffix to indicate a file to be included mainly because that the convention in many other programming languages. ‘h’ stands for header, or the prototypes of routines without their implementation. SystemVerilog does not really have the same need for header files, but people still the ‘header’ terminology to indicate a file that needs to be `included.