Simple question on DAX

I have following DAX that I have been using but I was wondering about this for a long time.

DAX =
IF( HASONEVALUE( tblTest[Name]),

         var CurrentTestValue = SELECTEDVALUE( tblTest[Field])

    return
          CALCULATE(
                    SELECTEDVALUE( tblMain[Rating]),
                    TOPN(1,
                             FILTER(
                                    ALL( tblMain ),
                                    CurrentTestValue <= tblMain[CutOff]
                             ),
                            tblMain[MaxTotal],
                            ASC
                        ),
                       All(tblMain)
              )
)

My question is:

1) What is point of All(tblMain)? I know that it would not make a difference if that part is there or not.