What is the point of passing a list of PsiFiles when creating a WriteCommandAction?
This is to support IntelliJ's really great Undo architecture. If you make edits in 2 separate files, you may have noticed that you can perform undoes and redoes in the two files completely independently of each other. In other words, each file has its own Undo history.
But, if you perform a Rename refactoring, that refactoring can touch a large number of files. And, you can undo the whole refactoring in a single go! In fact you can go to any one of those affected files and perform that undo, and as long as none of the affected files have any conflicting additional events after the refactoring, it will work.
THAT's the purpose of the file list passed to write command action: It lets the action know the set of affected files by the Undo, and it will generate a single atomic undo event across all those files, which lets you undo the multi-file action, and at the same time let you independently undo changes in other unrelated files.
If you have a giant action where it‘s difficult to predict the set of affected files, it’s probably better to not specify any files at all; that way the user can perform an undo to get rid of the whole change. But note that once you do this, a user can't go into one of their previously edited source files (unrelated to your action) and undo one of their edits since the global undo event is on top of everything. So if possible, try really hard to pre-compute the set of affected files.
When making new files, use the FileDocumentManager to get a Document and call setText on it rather than calling something like virtualFile.setBinaryContent(text.getBytes(UTF_8). This works better with the undo manager (something we learned the hard way when trying to make the New Project wizard's creation event be undoable.)