It isn't known when or where the use of herbs and spices began, but historians agree that they have been used to flavor foods for a long time. Herbs and spices have little if any nutritional value.
Humans can subsist on unseasoned food.
The answer to this question could be answered quite practically. It's often said that in the days prior to refrigeration, herbs and spices were used to mask the foul odors and tastes of rotten foods that were too disgusting to eat without the added flavor but were too valuable to waste. Herbs and spices also are said to aid the digestion of fibrous vegetables or greasy meats and to relieve the distress caused by eating too much. The aroma of seasoned foods also is believed to stimulate gastric juices, or to put it another way, to make your mouth water. Good food is different than food that is good for you.