All PCT applications are automatically subject to examination. The examination may be without interaction between the applicant and the Examiner (under Chapter I) or with interaction between the applicant and the Examiner (under Chapter II, that is, if a Demand is filed).
Briefly, filing a Demand (along with amendments and/or arguments) is a way of addressing one or more objections expressed in a negative WOISA, in an effort to obtain a more positive IPRP.
Also, filing a Demand permits central prosecution of the application before a single patent office, and can be used to reduce the number of objections that will need to be addressed later, during the National/Regional Phase, before individual patent offices. This is especially true for the European Regional phase, where further examination essentially continues from where the international examination stopped.