How backlinks help domain to rank?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Ammon Johns: The less it takes to get a link, the less that link tends to be worth, mostly because others will have spammed it into being worthless.
  
  The first thing to understand about backlinks is that `domain authority` is only useful for talking about how much potential `link power` is floating around that domain itself, spread all through by it`s navigation architecture.
  
  But you need to understand that a site with infinite pages, such as a social media platform with millions of users making millions of unique posts per hour, has just about zero link power on EACH of those pages.
  
  For the Domain Authority of a site like, say, Facebook to matter a darn to your link, you`d need that link to be a permalink in the navigation menu of Facebook, not something in a comment or post that is one of literally millions created in the last hour alone.
• 
  Sam Mah: Whatever your niche is, get the link from it. For ex. If your business is self storage, join the storage association. The .edu and .gov are still valuable if you can get a backlink.