Its not a gimmick, its the jackpot. Thats what it is, no doubt; you are collecting coins like crazy in that game. Its insane how many coins youre going to collect; the amount of 1UPs youre going to get are going to help you, yes, but its like hitting the jackpot because theres coins everywhere. And Gold Mario is able to throw gold fireballs, which allow you to gold everything else.
Theres basically every single opportunity to get coins that you could possibly think of, and you want to get them all
there actually is purpose to collecting all those coins, and Im not going to tell you what it is, but theres purpose, and you want to collect as many as you can." - Nintendo of Canada's Matt Ryan
When asked if the reward for collecting enough coins was a small bonus like a star next to your player profile, Mr. Ryan responded with the following.
I dont think that we would tease if it didnt actually mean something.