The main reason that women parking places exist is that parking areas are often considered as creepy. At some time they are completely deserted so you hear only the deep humming of the fluorescent tubes, every step (with high heels) announces loudly your presence (and that you are a woman, by the way), there is often bad lighting, few exits and both cars and the pillars are allowing people (in your vivid fantasy) to watch you without being seen. In short, eerie, scary, sinister.
It depends on the parking area if there are any legal means to enforce the reservation for women.
Once the owner admits that the StVO (the German road law) is valid (This is automatically the case if there are no effective borders to prevent access in contrast to e.g. multi-storey car parks), the owner cannot prohibit the usage by men. But if the owner has their own terms (as being said, all multi-storey car parks or workplaces with their own parking places) the current interpretation of the law permits the owner to exclusively reserve places for women and according to a decision in 2011 (LAG Rheinland-Pfalz, 2011-09-29 10 Sa 314/11) it does not discriminate against men.
The current state is that is not really enforced, but culturally accepted to give women a more secure feeling. These places are very near to the entrance, have good lighting and camera supervision. It is really more about a secure feeling, there is no statistical indication that attacks are prevented by those places nor that even attacks in parking houses are more common than in other places.
The actual usage is that it is impolite to use them as men if there are free places left, but once the places are full, they will be used, too.