The reason why it is important to add dried herbs and spices at least 30 minutes before the end if cooking is to allow the dried herbs and spices enough time to rehydrate and soften and release their full flavors into the dish that you're cooking.
Allowing the flavors to be fully released into the fish that you're cooking creates a more consistent and well integrated taste.
Adding the dried herbs and spices to the dish too late when cooking can result in a weak or dusty flavor.
And adding the dried herbs and spices too early when cooking can cause the herbs to lose their potency.
Dried herbs should be added at least 20 minutes to 30 minutes before the end of cooking to allow them enough time to rehydrate and release as much flavor into the food as possible.
Although it can also vary depending on the type of herbs that you're cooking with as to when you should add the herbs to the dish when cooking.
For more robust or woody herbs such as thyme, rosemary and oregano, that retain their flavors well during cool, adding these herbs early is often best.
And softer herbs like parsley and basil lose more flavor and structure when exposed to prolonged heat, so they should ideally be added later in the cooking process or as a fresh garnish.
And for dried herbs these herbs work best in dishes which involve longer cooking, like soups, stews and braises, where they have enough time to develop their flavor.