There are many benefits of sum types:

- Their values can be discriminated at runtime (it's depending on the implementation of union types whether that's possible. In C a union is just a structure large enough to hold all constituents)

- You can sum the same type (union(Int, Int) simply is Int)

- The tag breaks recursion. You can build recursive sum types because of it, e.g. 'a List = Empty | Cons('a, 'a List)