I've printed gears for my lathe:
Been running that way for several years now. No signs of wearing and it took the "ringing" noise out of the gear train. ABS print.
Assorted bits for building slot car tracks:
That's pretty much all PLA printed. Mostly because that;s what I started learning with.
RC cars/trucks:
Bits for audio:

And all kinds of other stuff. One of the other places I use it is restoring vintage sewing machines. They started using a lot of nylon gearing in teh 80's+ and once they crack, you have to make new ones. 3D printer is perfect for that.
I primarily print in ABS. Gave up PLA years ago. Abs is a little trickier to print with, but it has advantages over PLA.
3d printing is great for those little bits and pieces that tie all your other things together.
Places like thingiverse is a free to download repository where you can find lots of objects. Chances are if you need it, someone else does too and they may have already made it. TinkerCad is a good free site for beginners to play around building stuff without having to know CAD. Other than that, you pretty much need to learn CAD.
3D prited pars are not for everything, but there are places where it's just right.
My printer started out as a Creality CR10 V2. First add was a BLTouch sensor. Then an E3D Titan direct extruder made it a V3. An E3D Volcano hot end followed to keep up with larger diameter nozzles, then a stainless print bed and finally a custom firmware version of Marlin made it what it is today. Also has an enclosure to make for stable temps when printing ABS.