In the end carbs are just sugars of different complexity. For example add some saliva to starch and you got glucose. Hence white bread, pasta and even potatoes are really bad for your blood sugar.
You don’t have to avoid them, but carbs are not necessarily part of a healthy and balanced diet. Their only benefit is that they are cheap sources of energy. If you don’t eat carbs the body has to use fat to metabolize sugar which works just as well.
It is useful to avoid carbs for a diet, because you can remove a lot of calories from any meal by replacing them with mostly vegetables without reducing the portion size at all. Also it helps with cravings a lot.