My recipe for muffins tells me to fill the empty muffin tin cups half way with water...?!
You're right it's to keep the tin evenly heated. If your tin is full of batter,no need for water...
If you have empty tins the muffins that are next to those tins will get too hot and may burn before the other muffins are ready. Adding water to empty tins is a cheap and convenient way to prevent this from happening. If you are using every tin for batter then the water is unnecessary and your muffins should come out fine.