That way, if you have a flood in your basement that completely submerges the bottom shelves of your bookcases, you don’t lose anything especially valuable.
Early on Monday morning a water main burst in front of my house. This caused water to back up through the toilet and shower drain in my basement, leading ultimately to close to two feet of water covering everything. Fun! I discovered the flood at 3:30 in the morning, the clear moral being that it’s very important to keep eccentric hours. Had I been asleep like a normal person the water could have been in my living room before I noticed.
Never fear, though! It was only a very small portion of my extensive creationism collection that was destroyed, and I am sure that the volumes that were lost did not include anything that is not repeated in countless other books.