There are two ways to get people to your website, either you buy a click through Paid Ads or you earn a click through your content. In this way, SEO is a marketing investment, whereas Ads are a cost of sales, so you need to balance the two.
Investing in SEO means you’ll get more customers organically and spend less to buy them. Creating content that attracts customers takes time, but once it is established it will continue to work without further investment. Compare this to Paid Ads, where you get traffic while you’re paying for the ads, but when they stop, the traffic stops too.