Freedom Before Commitment
One of the main purposes of object initialisation is to establish invariants such as a field being non-null or an immutable data structure containing specific values. These invariants are then implicitly assumed by the rest of the implementation, for instance, to ensure that a field may be safely dereferenced or that immutable data may be accessed concurrently. Consequently, letting an object escape from its constructor is dangerous; the escaping object might not yet satisfy its invariants, leading to errors in code that relies on them. Nevertheless, preventing objects entirely from escaping from their constructors is too restrictive; it is often useful to call auxiliary methods on the object under initialisation or to pass it to another constructor to set up mutually-recursive structures.