There is no reason to have a belt other than looks. If you actually need it to hold your kilt up, your kilt doesn't fit right.

When dressy to formal I usually wear a waistcoat so no belt. The exception to is my doublet which requires one. If i just wear a jacket but no waistcoat then I will wear a belt. For casual wear I usually wear a belt unless I am wearing something that would cover the belt, like a sweater.