i don't understand something in color correction, when should i use Curves that changes Shadows/Highlights colors, rather than change the color itself by using for example selective color?
Photoshop, like most Adobe applications, offers 2 or 3 ways to achieve many things.
You can use Curves, or Levels, or Selective Color, or Adjustment Layers, etc.
When to choose one method over the other is entirely up to the user and their preferred workflow. In some instances one method may work better given particular subject matter.
Most often, there's no "absolute" reason to use one method over another.