I needed an L-bracket to hang a small router shelf off an awkward bit of skirting. The off-the-shelf ones were either too deep, too shallow, or sold in packs of fifty when I wanted exactly one. So I stopped looking and opened FreeCAD instead.
The design took about twenty minutes, most of which was me measuring the gap three times because the first two numbers disagreed. A flat plate, a perpendicular plate, two countersunk holes, a fillet on the inside corner so it wouldn't snap under load. Nothing clever. The point of the exercise wasn't to be clever, it was to get a part that fit the actual hole in my actual wall rather than the average of every wall.
First print was slightly out: I'd put the screw holes 2mm too close to the edge and the wall was thinner there than I'd like. Bumped them inboard, reprinted, done. An hour on the printer, PETG because it's near a warm router and PLA goes soft if you so much as look at it sternly.
This is the bit of 3D printing that actually earns its keep for me. Not the dragons and the Benchys, the boring one-off parts that don't exist for sale because nobody else has my exact awkward skirting board. It's holding the shelf up nicely. I will probably never make another one, and that's rather the point.