They're everywhere. Airliners have rivets. The pockets of your Levis® have rivets. Frogs make the sound, "rrriiiiiivvvet." That last example probably isn't applicable, but it kinda makes you wonder, doesn't it? Not only are rivets ubiquitous, they look super professional when used on a sewing project. Rivets also have a very logical purpose: they hold loads of thick layers together at points where it would be impossible to stitch with a sewing machine.
It's something you might not have noticed until you started sewing. The pattern on a printed fabric is not always lined up with the grain of the fabric (the up-and-down and side-to-side direction of the thread -- the warp and weft). Back when all patterns were woven into fabrics this wasn't a problem because the weave was the pattern. But now with most patterns being printed onto the fabric after it's woven, the 'registration' can be off a little. Or a lot. And that can wreak havoc with your sewing.