Why do you have to add sugar rhubarb?

You don't always need to add sugar to rhubarb. Rhubarb is a tart vegetable, and the amount of sugar you add will depend on your taste preferences and the dish you are making. If you are using rhubarb in a dessert, such as a pie or crumble, you will likely want to add sugar to balance out the tartness. However, if you are using rhubarb in a savoury dish, such as a chutney or relish, you may not need to add any sugar at all.