well, it lets you complete/challenge events at a lower level. If you have everyone at A rank, then you have already spent a lot of time with the game and have the corresponding levels and equipment. I only have 3 idols on A rank and on lvl 50 to 55 so far. If i want to beat the DL live for example, i'll have to use everything at my disposal. Of course, i can also grind several hours to compensate for that, and i probably will in the end anyway since i'd like to get the platinum trophy. I just mentioned kiramekirari because it's the song with the highest note count, and it's scoring potential due to combos is 1100 points higher than any other non DLC song, 1500 with chain bonus 100. To compensate for that you need 71 more omoide for example (in a song with an image type, 5 memories and duo omoide appeal), or 230 burst (assuming you use a costume with trio burst 3, more otherwise).
If your grinded enough to beat everything anyway, of course there is no point in trying to get the best out of your idols.