Thanks for the point you raise. I reckon that this is not clear.
Anyways, in the second example the ‘file’ parameter is defined so that, when the Observable emits, it does emit not only the lines read but also the name of the file read. This may be useful, for instance, if you want to log the name of the file read once the Observable is completed.
I hope that now the reason and meaning of that parameter is more clear.