This will help them remain distributed throughout the muffin batter when baking and keep them from settling at the bottom of the muffins. (thebusybaker.ca)
Yes, sugar would work fine as well, just make sure it gets dissolved so it doesn't settle at the bottom. (itdoesnttastelikechicken.com)
It may all settle at the bottom, but that's okay. (keatseats.com)