Hoarding is beyond my understanding. I think it's some kind of primitive "saving" instinct taken to some deliriously extreme heights.
there are different hoarders. some hoarders are like Mormons, storing close to a year's food & water supply.
other hoarders just can't bear to do the thinking involved in throwing away stuff. i have one friend i worked with 20 years ago who was hoarding old BMW's at the time (i was one of the few friends that would help him move them from one warehouse to another). over time he paid $100K+ for his "i'm gonna open a BMW parts store dream".
which his wife didn't support. i was an usher at their wedding. i watched them go through the divorce process - his wife didn't support the BMW habit.
after they got married the friend started hoarding - everything. not so much hoarding as, having trouble sorting through their junk + buying computer stuff because it was on sale (they gave me a 48X CD-ROM once when i was visiting).
now their divorce is finished, their eldest daughter is looking at colleges ... and the friend is going to a "clutter support group". something like that.
i have the same problem, partially. e.g. last year i was doing some composting, and stashed a little more ingredients than i needed, resulting in a very interesting collection of 50 pounds of oak tree seeks aka acorns ... which sprouted.
so i went into the garage and - threw them away.
that's the part hoarders have trouble with. throwing away stuff they probably won't need, that has no value (except maybe to squirrels & soil bacteria).
the friend with the BMW parts will go to a support group meeting (one way to meet chicks, now that he's divorced), and just leave the stuff sitting & accumulating.
another engineer friend has the same tendency - and he has 2 1/2 acres to store it on. he brings home very cool stuff (he is a key employee at a graphite composites manufacturer). his wife gripes when his junk-pile overgrows the 1/2 acre allocated to it, but other than that, it's not a problem.
bottom line is, i think a lot of people hoard things because they hope they will be able to use it in a particular project. they identify with that project, it might be a future business (e.g. my friend's BMW parts). it is too painful to deal with the reality that they may never start that business.
Jeez, maybe i could become a hoarder counselor.