I used to think a 3D printer was a luxury, now that I have 2, I know how useful they are. I use mine for all kinds of things. Making jigs, drill guides, templates, plane parts, the list goes on. Keep an eye out for others who are selling one they don't use.
There are plenty of files one can download of useful stuff to print. But the real magic is in learning to design your own stuff. For anyone looking to get into 3D printing, I would recommend you start by learning to design first. The design process has it's own learning curve. Myself I use Fusion 360, the free version, it does everything I need. But there are plenty of other choices out there, each with their own learning curve. You can start that now for free.