For me they're aspirational. I haven't played a pen and paper RPG in over 20 years, but like to imagine I'll find the time. With a baby on the way, in about 8 to 12 years I'll have a captive audience to play with, between her and my wife.
It's the same with our giant shelf of board games that are better with 3 players. We're playing the long game here.
You can start when they're as little as 4 and have decent results. I started with mine when they were 5 and 3 with Hero Kids and by the time they hit 6 and 8, they'd graduated to Pathfinder 2 (edit: albeit with a great deal of help/handholding)
10/10, would indoctrinate children with gateway drugs games again
I have the D&D 3.5 core rulebooks on my shelf in the nerd nook. I know that I'm never going to play 3.5 again, but it's the system I first got into the hobby with. It would feel wrong to get rid of my personal history of nerdship.
Even if I don't directly use each book, I might find ideas and inspiration in them that I can bring back to the games I do run. This has happened plenty of times. Besides, they can be fun to read. This goes for old books too. Numerous times I've adapted old material for new games.
My husband's computer recently died and he was very scared about the possibility of losing 40 plus gigs of digital RPG rule books. I did manage to save the drive so all was good.
There are so many advances in game systems since the Palladium system I use, so I have stolen shamelessly to make combat last less than 1 game session rather than the two or three under standard rules
The universe is good, the rules are ancient
I do have a shelf of books though for Rifts, TMNT, after the bomb
Hell, be an rpg hero and make it a lending library to your actively playing associates.
The only parts of my collection that I don't lend out are the absurdly difficult to replace box kits. Even then, I'll make copies of the material. My kid's d&d group plays 5e, but the DM has borrowed some of my 3.x books for ideas, and has (with full supervision because I'm a little protective) had access to my spelljammer box (before they redid it).
Besides, I may end up running a game again. Most of my regular players have expressed interest, and it's only scheduling that keeps it from happening. Shit, I might even do a game with the original rule materials some day, the way I used to do ad&d short games (a few months of a story, or breaking out a module) when we were playing 3.x
I have a vampire the masquerade v20 book and some supplements, not because I'm interested in running it, but because I think it's the most compelling vampire fiction ever written.