It happens mainly in North and South Kivu, where there are a lot of resources. And it's quite deplorable to see that the DRC has such a low level of development when it has such incredible resources.
The many armed groups, such as the Mayi-Mayi and the former CNDP, which are in those regions own and operate a number of small and large mines in the region. They must therefore have access to those mines and to airports so that they can transport the ore. They have to be able to control those mines. So there are often wars and important economic issues concerning the wealth of the chiefs to control these regions and operate the mines. Nothing must disrupt that situation. If it is enough to say that a given area belongs to them and to rape 300 women in a region to consolidate their power and control, they'll do that. It's a monstrous and radical way for certain individuals to affirm that they are the chief and that all that belongs to them.