I save packaging to make new things with. I don't want to be a hoarder, lol. I do go through my "recon treasures" all the time. I use a lot of chipboard so I keep a nice supply of chipboard. I keep it at a set amount. It's not like my DH is going to stop eating cereal anytime soon so I am in a good place. I do save acetate. I save unusual looking packaging like pretty backgrounds, interesting plastics, etc.. I do a lot of mixed media work. Interesting items are a goldmine for me.
A really great example I bought these cute bird embellishments the other day. The packaging they came in had a beautiful embossed design of the birds. Then the inside of the package had another piece of plastic shaped like a bird mold. The plastic itself is excellent quality and I can use it for a resin mold. I would have paid a fortune to find a mold like that.
I keep my treasures in a big box specifically for my found items. I noticed when I started saving items I started saving money too from buying similar items at the craft store.