Curve presets are to photo editing what sauces are to cooking. The right curve preset can enhance a photo, shift its mood, or take it to a completely different level. Good curve presets can help you to define your photographic style and can become your unique signature. Of course, even the best curve presets won't make you a great photographer (as a good sauce won't make you a good cook), but they can add a new artistic dimension to your photos. Creating curve presets, like mixing your own secret sauces, requires experience, skill, and taste. But if you want to create your own curves, you don't have to start from scratch.
The Daily Curves set offers 12 carefully-crafted curve presets for digiKam and the GIMP that you can apply right away, or use them as a starting point for making your own curves. The accompanying README.md file provides brief descriptions of each curve along with an introduction to curve editing.