Some recipes have CofT - some don't. What's the difference?
CofT gives the egg whites volume and stability. I don't know the exact science but it's one of those rules that I automatically follow adding it my egg whites when making both chiffon type of cakes and IMBC.
Where I live it is hot and humid 80% of the time. I had wondered if the cream of tartar helped the meringue buttercream to stay fluffy when piped.