A U.S. divisional patent application application can be thought of as a “child” application of an earlier filed “parent” application. The filing of a divisional application often occurs when the parent application contains more than one invention.
If the parent application claims more than one invention, the USPTO may issue a restriction requirement. The restriction requirement requires the applicant to elect among the designated inventions in the application. The election of one of the designated inventions may mean the others will not be considered. Therefore the applicant has the option to file one or more divisional patent applications directed at those other un-elected inventions. The patent office wants to make sure that patent applicants are not trying to claim multiple inventions in one application because they generate fees, at least partially, on a per application basis. Divisional applications can also be filed without a restriction requirement being imposed by the USPTO.
The divisional application, if properly filed, will be recognized as having the filing date of the parent application. Therefore, a divisional application is not disadvantaged by its later filing.
The Manual of Patent Examining Procedure provides that a divisional application is “a later application [e.g., a child application] for an independent or distinct invention, carved out of a  nonprovisional application (including a nonprovisional application resulting from an international application or international design application),  an international application designating the United States, or  an international design application designating the United States [e.g. the parent application] and disclosing and claiming only subject matter disclosed in the earlier or parent application [e.g. no new disclosures]. MPEP 201.06.
Therefore, no new subject matter can be added in a divisional application. Rather, the divisional application merely claims an invention that was already described in the original parent application.