This is important because choosing slowly absorbed carbohydrates, instead of quickly absorbed carbohydrates, can help even out blood glucose levels when you have diabetes.
Glycaemic index looks at foods in isolation, whereas we tend to eat carbohydrates as part of meal. Things, such as cooking methods, ripeness of fruits and vegetables and the fat or protein content of a meal will affect the glycaemic index of a food.
It's important to note that if you were to restrict yourself to eating only low GI foods, your diet is likely to be unbalanced and may be high in fat and calories, leading to weight gain and increasing your risk of heart disease. It's important not to focus exclusively on GI but to think about the balance of your meals, which should be low in fat, salt and sugar and contain plenty of fruit and vegetables.