Why use warm buttermilk? Never done that before with waffles.

Yes, just as Gleaner said, you just don't want the buttermilk to be cold so it doesn't cause the butter to instantly harden.

I think it's because the buttermilk is added to melted butter and using cold buttermilk would harden the melted butter. Alternatively, you could let the melted butter cool and use room temperature buttermilk.