One particular joy of 3D printing is the discovering some potential quality of life improvement, searching it up on Printables, Thingiverse, or any other 3D model repository and — lo and behold someone has designed a model for that! You pop the file into your 3D printer – maybe with a quick stopover at your slicer – et voilà, problem solved.
Sometimes, though, you have a need too specific to be solved by a premade design.
@harris modifying existing models is a ton of fun and super useful. One tip for people without much cad experience is that you can often make small modifications directly in your slicer (for example size alterations or even adding holes!) and for more complex modifications m, Tinkercad makes things super easy.