Consider this situation. You have agreed on a minor suit as trumps. You are not playing any form of keycard ask other than 4NT as Roman Key Card Blackwood. You have extra values and think a slam is likely, but have only one key card and the trump queen.
What will you do if you bid 4NT and the response is 5♥?
My suggestion is to use 5♠ as a relay to 5NT. You can then pass with this two-loser hand, or bid on to promise whatever 5♠ would have otherwise meant.
Of course this doesn't matter if you adopt optional minorwood.