I've used to have a very bad hoarding problem. Bike parts, magazines, computers and electronics, tools, clothes, etc.
If there was a space to put it in, there was a Rubbermaid bin with a label just waiting to be filled. I had a little bit of everything, and everything had its place. I did this for 8 years. I even moved it all from one apartment to another, twice. None of my friends questioned the usefulness of it all. They just helped me move it because I was going to give them beer and BBQ.
Then I lost my job and moved across the country. Packing everything you own into a 6 x 12 x 5 foot trailer makes you question the reason why you keep things. I threw away most of my hoarded materials. I've since trimmed things down even more, and I'm slowly chipping away at the last of my excessive stuff.

I used to hoarde because I was very poor. Two jobs and still couldn't keep up on the bills kind of poor. Having mild Asperger's and obsessive compulsive tendencies don't help with it, either. I kept everything because I knew I had backups, just in case. I couldn't afford to replace anything if I needed to buy it new, but I was really good at fixing all sorts of broken stuff.
I don't need to hoarde any more, and it's a tough thing to get over if you've done it out of necessity in the past. The mental block is there when you go to drop off a box of stuff at the book donation or the Goodwill... What if the one I've got now breaks? Maybe I'll just keep one. And then it all starts again.
I'm allowed to keep my one box of essential parts, and I go through it and make sure that I don't need to swap out anything that's in there for something new that I've found. No more, though.