It usually costs more to create something yourself than to buy it at the store (see "economies of scale" — mass-produced items can take advantage of custom jigs and specialized knowledge). However, there is one area that self-built items excel — you can make them to a custom size.
in woodworking, the most common example is a shelf or desk that you want to fit in a very specific (usually small) space
custom kitchen drawer organizers — There's a lot of wisdom in this video! The author has gone through several iterations over two decades, and has worked out most of the design kinks.
I've often found that polycaprolactone can create form-fitting pieces that can attach to something else, and that it's very likely that NOTHING like that piece exists on the market, other than a custom-designed 3D-printed piece.
Of course, most of the things that people use 3D printers for, are things that don't exist anywhere in the market. 3D printing is usually one of the most expensive options... but people still choose that option often because they want extremely rare or bespoke pieces.