SEO if implemented correctly will let you know how many unique visitors, how many repeat visitors and referral traffic your site is getting.

Unique Visitors – This is the total number of individual visitors that have visited your site in a specified period of time. This doesn’t include repeat visitors, resulting in an accurate assessment of how popular your content is. This tells you how well your campaign is driving visitors to your site.
Repeat Visitors – This tells you how many visitors have visited your site more than once. This tells you how attractive and resourceful your content is if people think enough of it to come back. If you don’t have enough repeat visitors, you need to add more valuable content worthy of a repeat visit.
Referral Traffic – These are non-search engines sites that send traffic to your site. How do you attain more of these? Inbound links! Start featuring attractive and resourceful content that others share on their own site or blog.

Using SEO to measure these metrics will let you know what parts of your website are working and which ones aren't so you can make the changes accordingly.

Source: www.quora.com