In anatomy, lacunae are the small gaps within the bone matrix; in the gaps reside osteocytes. This sytem of cavities (lacunae) with residing osteocytes are connected through a system of minute canals (canaliculi).
In the literary world, authors sometimes utilize the tool of narrative lacunae in order to highlight the limitations of language in capturing the panorama of human experience and expression. Authors who use this literary device want to emphasize for their readers the ephemeral power of language in defining motives and fate. In Gunter Grass' Cat and Mouse, for example, we don't really know what happened to Mahlke in his final act of diving down to the minesweeper. Mahlke dives down with the cans of pork but he does not bring with him the requisite can opener. We know that he is disillusioned with his life and with the futility of all that he has tried to achieve. All his life, he has tried to make up for his grotesque appearance: an enlarged and "monstrous Adam's apple." After earning Germany's Iron Cross for valor in combat, he is refused a speech in front of the student body by his former headmaster, Klohse.
After Mahlke disappears, Pilenz hypothesizes that Mahlke, the "mouse," has fallen prey to the "Cat" we all fear in this life: the insidious tentacles of an imperfect societal structure with all its suffocating expectations and requirements. Did Mahlke disappear in order to make peace with his own psychosis? Or did he simply drown, an act of suicide as a response to his challenges in life? This lacunae of nothingness serves to illustrate for us the realities of our human experience: in real life, we may not know either. It is this lack of perception that sometimes hampers our search for answers. How many of us have experienced this when a friend moves away without saying anything to us, for instance? We may be hurt, and left feeling helpless; the sad truth is that we may never find out why he/she acted in such a manner. The narrative technique of lacunae just portrays for us this reality.
Hope this helps. If you are interested in literary lacunae, you may be interested in this work: The Dark Matter of Words: Absence, Unknowing and Emptiness in Literature by Timothy Walsh. It's heavy reading and it's ok if you do not agree with everything that is written (I didn't); the book just gives you a little bit of an insight into this literary device. Above, I have given you the most basic reason as to why Pilenz would have used this technique.
A preview of the book is available here.