Recent example -- I got a Husky work table with a router plate insert (to make it a router table). I have an old Craftsman router. No one makes router table plates that have holes drilled for this model router. So I modeled and printed an adapter plate to screw onto the router, with threaded holes matching holes on the table's insert plate.
A previous example, I needed a plastic ring to hold magnets for a cadence sensor for my E-Bike kit. The kit came with a magnet ring for a mountain bike, but I have a road bike. Couldn't find anything suitable online for this particular bicycle. A couple hours of modeling, and 3 hours of printing later, I had the part that I needed.
As for plastic waste -- it isn't as much as you think. The slicing software (which converts a model file into gcode) lets you put in a fill percentage. Default is 15%, and upping it to 35% makes a fairly strong part. It uses a honeycomb fill pattern, which is strong yet uses minimal material. I'm still on my first two 1kg spools of material (and the slow speed of creating something means that you don't go through that much filament that fast).
Another project was making small clips that I used to mount a piece of plexiglass (from a big box store) over the TV, as protection from the grandkid that we are raising (it is surprising how delicate modern TVs are).
So in short, 3D printers are really good for making small parts that you need for various household projects or hobbies, and a $20 spool of filament lasts a long time. And they get really useful when you design your own "things" to print.