[masklinn]

Unions can be used as a somewhat safer (not safe by any means but safer), more flexible, and less error-prone form of transmute. Notably you can use unions to transmute between a large type and a smaller type.

That is essentially the motivation, primarily in the context of FFI where matching C's union behaviour using transmute is tricky and error-prone.

[randomNumber7]

There are rare cases where all attributes of the C union are valid at the same time. Say you have a 32-bit RGBA color value and you want to access the individual 8 bit values. You can make a union of an 32 bit int and a struct that contains 4x 8 bit integers.

Also you can manually tag them and get s.th. more like other high level languages. It will just look ugly.

[Animats]

Yes. I once wanted C unions limited to fully mapped type conversions, where any bit pattern in either type is a valid bit pattern in the other. Then you can map two "char" to "int". Even "float". But pointer types must match exactly.

If you want disjoint types, something like Pascal's discriminated variants or Rust's enums is the way to go. It's embarrassing that C never had this.

Many bad design decision in C come from the fact that originally, separate compilation was really dumb, so the compiler would fit in small machines.