It has nothing with mutable state, actually. I can use this approach with immutable state as well. It's just another value "uninitialized" for property. Like `null` but throws faster (and potentially separate from null, because null could be a valid value).

I'm using it for SQL queries. My query returns subset of some column set. It's not practical to define a separate class for every query. So all queries for the given table returns the same class representing all columns in a given table. But if one method returns subset of those columns, other columns contain uninitialized value and should not be read. Any attempt to use uninitialized value is a bug and must be caught as soon as possible.

"It's not practical to define a separate class for every query."

I'm not sure it's really impractical, at least in my experience. You may not need a separate type for every query, as long as you are willing to have some queries select a few more columns than you actually need right now; and you don't need separate classes, just separate interfaces, which makes the amount of extra code you need smaller.