Back in 1997, when I was simply an avid reader of PC Gamer magazine each month and not its editor, I picked up the July issue and, checking out the CD Gamer demo disc that was supplied with it, I immediately spotted the name 'Fallout' on the cover.
It sounded super cool to my teenage ear, so I installed the demo, saw the now iconic key art of the power armor helmet for the first time, and the rest is history.
On the strength of the Fallout demo I bought the full game, and then proceeded to buy Fallout 2, Fallout Tactics, and every 3D Fallout game since (Fallout New Vegas is my favorite of those).
And that's interesting because, as once noted by Fallout lead designer Chris Taylor, he 'didn't think the demo did a very good job of portraying the actual game' and that for him 'the most memorable part about the demo was when Brian Fargo played it for the first time he was killed by a hernia (critical failure on a Strength check) in the first area trying to open the manhole cover to the sewers.' For me, though, the Fallout demo was a key part in why I picked up the game.
The demo is not long by any means, and like the original main Fallout release, it's a bit unfinished/janky here and there, but it's an interesting window into how the original game was marketed.