If you think about it, it only makes sense for nikkes to lactate. Sure, they can't get pregnant but they do consume food, aka nutrition. This nutrition, while not as important to them (if at all, I can't remember), could be very valuable to their commander.
Imagine if you will that you're on an extended expedition to the surface, you're out of rations and can't contact HQ. Luckily for (You), the nikkes under your command have been consuming a bit of food during the expedition and much more before you set off in the first place. How would you access it? Regurgitation?
No. Breast milk. Nutrition, your very survival literally accessible on tap of the teat of your buxom nikkes.