If you want to achieve growth online, you can do so in a variety of ways: acquire more visitors through advertising and SEO or by convincing more visitors to convert.
If you manage to increase your conversion rate (and/or average order value), the ROI on your advertising and SEO efforts also increases. This allows you to invest in advertising channels even more.
Note that you also learn something about your prospects/customers and that these insights can be used in other marketing efforts.