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.

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.

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.

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.

A good DM knows the rules for the system he is running, a great DM steals from every RPG in arms reach.

You could say that about literally any library of books, not just RPG books. Even if I myself may not read the books regularly, I still read them at some point, OR potentially I’m like a torchbearer, I’m just taking care of all these books for the day when somebody else discovers them and is able to read them. I may be dead and gone by then, but I’m doing my part to keep the books and the hobby alive.

I can't afford that many paper books, man, sorry

Do I have every book for DND pirated as a PDF so that I can reference them, though? maybe

Do I have a digital backup of the Cyberpunk books? totally

Do I have an esoteric collection of random magic items and mobs I've seen as I browse over the last 15 years? oh fuck actually did I accidentally delete this folder I literally just realized as I typed this oh no

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.

Looks wistfully at L5R, Shadowrun, Deadlands, and a shitload D&D 3.0