Some licenses require it. Unrestricted, Documented, Separate . Packaging of Modifications specifies how the source code tree will be maintained, and the end users responsibilities to that tree of code. Source code is shared to be modified. Licenses differ on how modifications may be distributed.
Some licenses require that at least the modifications be documented.
Other licenses require that the modifications be packaged separately, though this practice is often considered an impediment to cooperation. OSD does not permit discrimination based on the context in which the modifications will be used. None, Binary only, Source . Some licenses prohibit charging a fee for binary only distributions, or other derived works not including full source code. Some licenses place restrictions on the circumstances under which a fee can be charged for the source code itself.