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I agree most printables site are full of variations of useless crap, but there are really well designed models (not just good looking) out there and I'm learning heaps just from examining the models to see where they've used curves instead of sharp corners to provide a structurally stronger region that can take more strain. being able to basically prototype myself at home gives me a better appreciation for well engineered parts. It may not be too applicable to metal working but wood workers could benefit too as using a filament printer you are laying down in layers, get the temperature wrong and the layers dont stick together so well plus the layers themselves provide a weak plane for force vectors just like wood grain direction matters in structural constructions.
I'm yet to make good use of the printer for astronomy. I made a part that keeps my telephoto zoom lense at full extension as its prone to shorten itself from its weight when tilted upwards. I just started printing rubber filament yesterday and its great, I'm going to make a larger diameter focus wheel to fit over my existing one for a bit better focus control. You can print out bahtinov masks, mounting parts for lasers and finder scopes, clipon frames to add cooling fans and keep cables tidy etc.
Granted these can all be done other ways and mostly once done are there forever. Printed plastic is not really suitable for precision and load bearing that mounts can require. but even something silly like glow in the dark filament to make clip on bands to put on the bottom of your tripod legs is a simple and useful thing you can easily knock out. Its up to your imagination really and if you're a diy kind of person.
I'd love to have a lathe and oxy set too but now with one hand metal isn't practical for me. Hey you could print sacrificial plastic parts for working on metal maybe? Guide rings on parts when lathe turning, bright colour plastic under/inside a metal part to make it easy to know you've drilled/milled through? corner angle brackets to hold pieces it position so you can spot weld in position, or even just print a design in plastic to test fit before commiting to cncing a block of metal.
Plus you could build a replacement for that damn little plastic thing holding your power tool handle together that you cant but a replacement for without buying a whole assembly. A 3D printer and some design skills is just handy for those nuisance repairs around the home where glues may not be appropriate but you need to get something decent in place, maybe make a brace for a cracking part to reinforce it?
Its just another tool in the workshop basically and for those interested I've seen many buy a cheap 3d printer and just have nothing but troubles getting anything to print reliably but finally we are starting to see them emerge under the $1k mark.
plus decorate the xmas tree with printed crap and print gifts when you cant be bothered to buy
All this advice is for anyone to mull over if they are tinkerers in any way, btw. I'm just so happy my new printer just prints every filament I throw at it, even the old stuff I thought had gone "off".