There is a moment every 3D printer owner reaches where the machine starts printing parts for itself. Mine arrived a few weeks ago. The first thing I printed that was genuinely useful was not a model or a gadget. It was a fan duct to fix a cooling problem on the printer that printed it.
It feels faintly absurd, and it is also one of the best things about the hobby. The stock filament guide rattled, so I printed a better one off Thingiverse. The spool holder bent under a full roll, so I printed a braced replacement. A cable kept fouling the gantry, so a five-minute clip solved it. Each fix is the machine improving itself, which is a satisfying little loop to be standing inside.
The catch, and there is always a catch, is that you can disappear into it. I have caught myself printing the third revision of a knob bracket when the part I actually wanted to make has been sitting in the slicer untouched for two days. At some point the printer is meant to make things that are not more printer. I keep meaning to get back to that.