It makes sense often to have a field that could return, for example, an Int or a String. It would be convenient to be able to use a resolveType-like mechanism to determine how the value is serialized. This would be even more useful for custom scalars.

The specification currently says: "GraphQL Unions represent an object that could be one of a list of GraphQL Object types", which basically means that unions are the same as interfaces, but without any shared fields. Supporting unions of scalars would further differentiate unions from interfaces.