When one's kids get their own places, they tend not to have a lot of space. Few kids move out into 5000 square foot houses or apartments with giant closets and storage rooms. For both practical and sentimental reasons, their stuff tends to hang around, collecting dust and taking up potentially-usable space.
Even when you love them to death, at some point you want their stuff out. Baby birds have to fly. They will accumulate their own mountains of stuff, in time, and the cycle of stuff will repeat.
I remember when my Mom advised me to empty my old bedroom of anything I wanted to keep before it disappeared. I thought that sounded very cold at the time, but I now realize that it wasn't. They had done their job, and done it very well indeed at considerable sacrifice.
My old bedroom was destined to become a guest room - and a room where the grandkids could stay to visit. However, to this day it has a large ceiling to floor bookshelf with my old books on it.