I consider them as "accessories" . I have been making my own AC cords and interconnects for years now with great results. There’s some good DIY recipes out there that will cost you a fraction of a commercial available cable but still, it is not as cheap as one would think. I have also tried a few cables from famous cable brands with mixed results.
Cables can of course help to bring out the best of your components but they will never be a better solution than a component replacement. One lesson learned is to let a new cable burn-in long enough (>100 h or more) so you can really judge its effect on your system. What your hear on day 1 of using a new cable in your system will be very different after a few hundreds hours of usage. As someone pointed out, ROI on cables is a diminishing affair so there’s an equilibrium one must think about before spending too much on this adventure.