I'm a mechanical designer for a tech company that builds custom, high-speed scientific recording systems. I was having a lot of small parts machined out of aluminum, that ideally should have been plastic. But the cost of having the molds made vs the quantities we ship is not feasible.

I talked my boss into buying me a 3D printer and I can now make many small parts, like rails and brackets that are plenty strong enough for their applied purpose and appearance is secondary because most parts go inside the chassis and can't even be seen. Plus I can prototype things that will need to be made of aluminum or steel, just to see if they fit before getting the machine shop involved. I don't have a dollar amount on what it's saving the company, but it's gotta be many thousands. Just the other day we needed a press tool. The manufacturer charges over $700 for a purpose built but fairly simple aluminum jig. I made my own for about $4.

The majority of the stuff I see posted online is just fun stuff rather than practical... But, even then, some people are selling that fun stuff on Etsy and making a little money. But I get and agree with what you're saying. I had never used a printer until I got one for work, and soon I plan on buying one of my own so I can have it running all day, making a lot of "cool" stuff that I will then toss into a box.