Natures increase or decrease stats by 10%, not "one or two" as you seem to believe. This matters especially with Speed, as the nature means the Pokemon cannot outpace and potentially counter Pokemon it ordinarily could, which affects its performance and utility in competitive play. For example, Sassy means that Lycanroc would not outspeed a base 80 Pokemon with comparable Speed investment, which is an important benchmark that it would be a shame to miss out on.