Quite simply, social SEO is important because it allows social properties to rank for relevant key phrases in the search engines.
Having social media accounts rank for a key phrase, or even better - a brand, can help to accomplish a few things:
1) Increase brand awareness
2) Aid in online reputation management
3) Positively influence click through rate of other search results
Outside of these benefits, social SEO helps to actually make social profiles and pages more interesting and useful to users.