union types are great. But alone they are not sufficient for many cases. For example, try to define a datastructure that captures a classical evaluation-tree.

You might go with:

    type Expression = Value | Plus | Minus | Multiply | Divide;
    
    interface Value    { type: "value"; value: number; }
    interface Plus     { type: "plus"; left: Expression; right: Expression; }
    interface Minus    { type: "minus"; left: Expression; right: Expression; }
    interface Multiply { type: "multiply"; left: Expression; right: Expression; }
    interface Divide   { type: "divide"; left: Expression; right: Expression; }

And so on.

That looks nice, but when you try to pattern match on it and have your pattern matching return the types that are associated with the specific operation, it won't work. The reason is that Typescript does not natively support GADTs. Libs like ts-pattern use some tricks to get closish at least.

And while this might not be very important for most application developers, it is very important for library authors, especially to make libraries interoperable with each other and extend them safely and typesafe.