I have all my stash listed (yes, all, it’s easier when you do it from the beginning!) for a few of reasons.
It helps me tie yarn to a project easily, which then let’s me go back to see how I’m supposed to wash something if I need to, for example. I’m never going to remember just exactly what yarn it was I made it from to look it up in the database later.
It’s a far easier way to browse through my yarn to “shop” for a project than pulling it all out is.
I can keep track of what I paid for stuff for tax purposes. (Designing and charity work.)
I can see what yarn I have just sitting around that doesn’t have an assigned project as yet. And just how much of it I have so I can go pick out a project.
I have semi-precise yardage numbers on what’s left of all my yarn that I’ve used. Which makes it possible assign it to a pattern that needs a stripe or contrasting ribbing or something.
And mostly, to remind me that I have 150+ different yarns (yarns, not skeins) in my stash right now, so - no - I’m still not allowed to go buy anymore.