The advantages of ensuring your site is properly SEO optimised are:
Higher, more accurate traffic with a better level of quality. More people will see your content and you will be more trustworthy as you are recommended more highly by search engines.
More sales from your goods and services. More advertising revenue and affiliate money if you’re a blogger like me.
Improves brand awareness - great for large and small businesses. You want to overshadow your competition where possible too!
Less time wasted making content no one can find.