Underworld Academy

1. OD&D is a working RPG which was run and enjoyed by so many people that it spawned a genre that was instrumental in producing your personal favorite RPGs, and it continues to work for many people. 
2. OD&D works differently from (say) third edition, Fate, or Dungeon World. Not just in the details, but in very broad assumptions. These functional differences matter; even if they do not matter to you, they matter to me and others.
3. RPGs live in different parts of design space. They do not simply move forward on a timeline of technological progress, where whatever big publishers or the "indie" scene produces simply makes past RPGs obsolete. Rather, fashions change and different corners of design space are explored or returned to.

OD&D defined the RPG genre. It looks like "RPG" became popular as a generic term for games like OD&D, so it's kind of prototypical. If you want history, you can go back as far as you want in Blackmoor, Braunstein and wargames like Kriegsspiel. If you want a more conventionally organized and quickly playable game, you can read B/X. But OD&D is the first full-blown published RPG, the one that inspired and/or provoked the rest.

At the same time, OD&D is not really what we expect RPGs to be. It represents a point in time where games designed on the lines of third edition didn't even exist yet. Now, even when people want to do "old-school," they start with and revert back to standard advice and prevailing styles and whatever they learned RPGs from. They write books full of stuff RPGs are supposed to have. OD&D shows other ways people made RPGs work, other models for gameplay. It provides a fresh angle on the subject, way out of the norm.

I take OD&D as a point of departure, or a point of reference - the RPG equivalent of the biological "model organism" like Mendel's peas, fruit flies, C. elegans, white mice. It's a point in RPG design space, indicating the existence of a larger region of RPG design space, and the presence of viable and interesting designs in that region. And you can study OD&D to see the way this can work, even though it has to work in quite a different way from third edition or whatever.