One makes a site where you don't log in but via browser fingerprinting so we can pop up shit like 'Welcome back, Carol'... (to be a bit intentionally spooky). Of course I'd need to already have compromised some 'fraud-prevention' racketeers like SEON or Kount to make a global fingerprinting reverse-lookup. Pretty high collision rate I'd expect. I expect Amazon, Shopify etc. use their own db from account data you give them...