Why Get a 3D Printer.
The short answer to this is... because 3D printer.
The entire concept of 3D printing is fascinating to me. To oversimplify it -- you want something? you print it?
More practically though, I enjoy tinkering and building things. Stuff that I can't get commercially, and items that are available retail that I can make cheaper. I also enjoy the process of planning, designing and building as much as (and sometimes more than) having the finished project. A 3D printer will give me greater ability to manufacture specialty parts for future projects.
As well, I have two sons and a 3D printer will give them some experience designing and printing things. Maybe it'll give them an edge in the future... maybe not.
But most of all, for me, the reason for buying a 3D printer is... because 3D printer.