My grandmother used to save everything starting from plastic bags, strings, plastic food containers, used nails and screws to electricity, water, heat and money. I suspect it is because when she was young there was a serious lack for everything from food to building materials.
I can see the same attitude in my parents except that they hoard a bit more expensive stuff like books, tools, electronics, music containers and furniture.
I used to collect electronics, music, comic books and MTG-cards but at some point I just stopped. The need to collect more stuff that I had learned from my elders wasn't useful anymore. It is also apparent that some companies are trying to market and exploit this behaviour for their own gain.
I still own too much stuff to fit in my apartment. I find it extremely hard to throw away 'perfectly good' stuff if I feel that it could be useful to someone. Yet it is quite laborious to sell it or to find someone who needs it.