These two items are similar in several ways but have some obvious differences. A jacket is heavier and mainly serves as the outermost layer worn. Many jackets (not all) have increased wind resistance and rain resistance. A hoodie, while comfortable, is not usually wind resistant or rain resistant. It makes sense, then, if you’re layering, to have a hoodie act as a middle layer that increases warmth due to the hood and long sleeves. Slap a wind/weather-resistant jacket over that hoodie, and you’re ready for colder, windier weather.