SEO is the biggest traffic source to many websites, that's why the link-building is so important.
But there are exceptions, one of my stupid site (similar to buzzfeed) get 90% of it's traffic from social media platforms. The best part is, I have done zero linkbuilding on that website. But I regularly share things on Pinterest.
One of my friend, get her 98% traffic from her YouTube channel
SEO is just a competitive method. Google gets only 30 percent of total direct traffic of the internet. So, there is gold in some places. Go and find them.