To perhaps lean more into why complex carbs are useful:

Your body can’t really not digest something you’ve eaten. Once it’s in your stomach, it will be broken down and gets put into your blood. With the simple carbs, you get a lot of blood sugar very quickly and your body then has to deal with that. It does so by producing insulin, which tells the rest of your body to take sugar out of the blood. It’s put into either a limited, temporary storage (glycogen) or, once that’s full, into more permanent storage (body fat).
Eating lots of sugar can also lead to your body producing too much insulin, which will cause too much sugar to be taken out of the blood, so you often have a high and then a crash/low after ingesting sugary foods.

Ideally, you want blood sugar to always stay at a reasonable level, where it can supply your brain and muscles, but where your body does not have to start storing lots of it. And that’s where complex carbs are neat, because they don’t get broken down all at once, when they’re in your stomach/intestines, meaning their sugar enters your blood at a more sustainable rate. By eating them instead of sugar, you’re less likely to put on fat and less likely to have a crash.