I think, there can be many situations:
- You want to pass some parameters from form
- You want to manipulate the form (for example refresh datasource after action is complete)
- Something other
But in all the cases depending on particular form is not a very good idea.
In first case you can set parameters from code using parm methods, or, better pass parameters using the Args class
In the second you can cast Args.caller to some interface that contain all the methods you want and manipulate the form using that methods (see \Classes\SysFormRun_doRe usages for example)