They are immediate
As anyone with a website knows only too well, it can take a long while to rise up the search engine rankings for your chosen keywords. It's often a two steps forward, one step back process. You have to do many hours of keyword research, content writing, building backlinks and on-page tweaking. And even when you've done all that work things often don't pan out as you'd hoped.
It's hardly surprising it would be this way, of course. Your pages are being ranked by a giant computer, not a person. So it's really out of your control -- even though you can make educated guesses about how best to approach things.
If you get into social media, on the other hand, you can draw targeted traffic pretty much from day one or soon after. Sure, that takes some work, too. You have to find people, engage with them, share your own stuff. But it's certainly quicker than SEO.
They are human
At the risk of stating the obvious, sites like Google Plus, Twitter, and Facebook are social in the truest sense of the word. They are populated by countless real people who are genuinely trying to connect with each other and form communities.
And if you become part of these communities, they give you a sense of belonging. The interactions you have in them are often warm and friendly, much like those that occur in real life. They're just mediated by technology, that's all.
Of course SEO is not entirely devoid of humanity. You are still trying to connect with other real people by working out what they are searching for and how to appear in those searches. So you have to consider their wants and needs. However, you're not often actually engaging with real people. If you spend all your time on SEO you can easily wind up feeling alienated, as if you're floating in space.
They offer quality
I don't think there's much doubt that search engine traffic is still best for conversions. But traffic from social media is still very good quality.
The Twitter followers who are clicking on the blog links you share on the site, for example, are already highly targeted. The very fact that they're following you means they have some respect for you, and share your interests, after all. So the odds that they will become regular readers, and maybe even buy goods or services that you are promoting, are quite high.
You can further improve the odds of this happening by repeated engagement with them on Twitter itself. People buy from those they trust, after all. And the more they communicate with you, the more they trust you.
They help with SEO anyway
The final benefit that you get from using social media is an indirect one. Google now records the number of shares, likes and plus ones your pages accrue. The more of these you have, the more positively your site is perceived by the search engine. It then rises up the rankings, gaining more targeted traffic.
This isn't a massive effect, but it can accumulate to become quite substantial. It's just one more reason you should regularly engage in social media activity.