SEO and SMO are a fundamental part of marketing your business online. If you do one without the other, your business won't do well enough.
Some think that all you need to perform well in search is a fast website with high-quality content and relevant meta tags. As you can assume, it's not enough. Google's algorithms take many signals into account, and social media is one of them. A study concluded that a larger presence on social media does gain a higher place on the SERPs. In other words, if your social media is optimized, you're likely to perform better in search as well.
To see SMO impacting SEO, you can search the name of a popular brand on Google. It's more than likely their social accounts will be listed within the first five results.