As someone mentioned below, the order of keys is undefined in JSON spec, but applications may rely on it anyway, and thus conversion to JSONB may lead to breaka

As someone mentioned below, the order of keys is undefined in JSON spec, but applications may rely on it anyway, and thus conversion to JSONB may lead to breakage. There are some other minor advantages of having exact representation of the original - e.g. hashing, signatures, equality comparison is much simpler on the JSON string (you need a strict key order, which is again undefined by the spec, but happens in the real world anyway).