The general reaction to 3D printing that I see at Splatspace is "oh, neat" followed quickly by some variation of asking what it's actually good for besides generating more plastic trinkets.
But those examples are usually too far from the experience or interests of those asking to be a satisfying answer. Here's a simpler, more personal use.
When my grandfather died among the bits of his belongings that I inherited was a gorgeous set of binoculars. They're not particularly fancy, but like many older things they're well made. The only exception being the cheap rubbery caps that fit over each lens.
When I got the binoculars, one lens cap had already torn and no longer stayed on the lens. The others were threatening to go at any time.
The binoculars don't need these covers. As long I store the binoculars in their case, it's unlikely anything will damage the lenses--they're solidly built. But as these were something my grandfather passed on to me, I wanted to do my best to keep them in condition.
And so I turned to 3D printing.
Like any hackerspace these days, we have a number of 3D printers. I'm not particularly good at CAD yet, but in OpenSCAD a cap is just one flat cylinder differenced with a smaller cylinder.
Simple as it was, it still took three tries to get right. The first was way too large; the second, though it fit, was tight enough that I worried removing it would break the lenses. The last try was almost perfect. Despite needing three iterations, the whole process took less than an hour.
Here's the final cap, on the camera, ready to go. Score for 3D printing helping maintain old gear.
On the offhand chance you need to print a lens cap for an old set of binoculars, you can find the design file on thingiverse.