Their main purpose is to close the product. Six screw bosses are used for this.
Bottom part and top part screw bosses have complementary shapes. This way the bottom part is positioned first and then the screws are used to press the bottom against the PCB and top part, keeping it from moving.
On the outside of the product, these screw bosses have larger cavities where the pads fit. As you can see on the section view, the screw head completes the bottom of the cavity, making a flat surface on which the pad can be taped. That way, the screw can not be seen once the pads are in place, but are easily accessible if you need to open the product without damaging it.
The ribs inside the bottom parts are here to improve the rigidity of the part. Their drawing was made so they don't touch any of the components on the PCB. A large section is left empty for the battery.