If you don't know WHY they existed, then you don't know IF they are STILL required today. Rightly dividing God's WOrd of truth works when it's actually tried....2 Tim 2:15. Either GOd meant what He said when He said ONE, or He forgot there were more.
I count four question marks (and it appears there should be a fifth), yet you mentioned "a question" and "two questions". What is your concern exactly?
Its rather simple really. If we have to do something to obtain salvation its no longer grace is it.