SEO, Anchor Links and Backlinks
Google, Bing and other search engines use anchor text (generally external to your website) to discover who is talking about your website and creating a "buzz" on the Internet about your wonderfully created unique and engaging content!
Be it on the social media platforms OR niche related websites, whenever the search engines "see" a link to your website, this is known as a "Backlink". It links BACK to you.
If you sign up with the different social media platforms, of course you create these links yourself and hope they get shared by others.
If your website is in a competitive niche, and lets hypothetically say that your website (while unique) might have all the characteristics to reach page one on the search engines for your keywords.... and yet there is a very similar competing website that in all other aspects is equally as good as yours, (so, all things being equal) - likely the one with the more backlinks from external/related websites will win and rank higher on Page 1 of the search pages.
So, the takeaway here is when in a competitive niche and there are similar websites, the more backlinks wins.
BUT.... Wait!!!! There's more!!!
:-)