Try them on drugs!
Seriously, though, for me, demos were/are an escape. A different kind of escape than a game. Sometimes you don't want interactive. The same way you will sit down and watch a movie or something. Just relax and enjoy a visual onslaught of things you've never seen before (or a variation thereof). Sometimes it will be the perfect combination of visuals and music, or just being impressed at new graphic tricks the coders came up with. For me, it's a whole lot more fascinating than a stock 3D engine wrapping bitmapped textures over yet another dungeon or cave wall. Sometimes, demos have a trance-like effect on me. Sure, some demos are the standard fare with the same ol' same ol', but a good deal of them offer fresh content, some groundbreaking, some funny, some displaying a smart design sense, etc.