This game is different in end-game compared to most. Most other mmo's have a level-based BiS for every class, and older gear is immediately garbage after a new update with a level increase. Gear is also class-specific in most other mmo's meaning your character only has to keep one or two sets depending on their role.
In ESO every set has the potential to be useful and all classes can use all sets so you tend to want to keep many of them in case something gets buffed or nerfed.
Now that we have transmutation though I am finding less need to keep the "perfect trait" sets since now you can pick up any traited version of it and simply transmute it.
Ironically the storage saved by not keeping those sets are being filled up by transmutation geodes, lol.

You don't know which sets you previously undervalued, or which will become useful next patch. I keep many, but not all, in various stages of completeness, particularly if they're BoP. Probably threw some good stuff away that could now be transmuted. I also got tired of re-crafting levelling gear, so one chest is dedicated to low-level crafted sets and hard-enough-to-find low-level Willpower / Agility jewelry. Next holiday XP buff event might see another character.
i'm a hoarder in eso, i'm a hoarder in real life, although i prefer " collector"
https://www.youtube.com/watch?v=ecnsGaIiXnA