You can make a tagged union in any language, and that can often be useful, but when it is a standard part of the language you prevent any accidental cases of:

You can make a tagged union in any language, and that can often be useful, but when it is a standard part of the language you prevent any accidental cases of: 1. failing to check for sigils, because nobody uses a sigil when you have sum types. This would have prevented a famous exploit in SUDO a couple years back where someone forgot to check for the special negative value on an input parameter. 2. Eliminating null pointer exceptions or anything similar. You can't forget to check, you can still choose to have this kind of error, but you have to do it explicitly. Consider something like a function to open and give you back a file handle. In many languages such a function will return a pointer to a file, or null if there is a failure, or perhaps throw an exception if there is a failure. If you neglect to check for null, or forget to handle the exception, you have a problem. With sum types, the natural thing is to return something like a Result type, which is EITHER a file, or an error. And you have to check which it is, to get the File out.