The concept behind the initiative is to “hide sites to find more of what you want”. It is quite common to do an internet search and get some results that are of no interest to you, using this new feature you can effectively filter these websites out of the search results, therefore having a results page specifically tailored to your wants and needs.
Although anyone that does a Google search will see that there is an option to block a website, the functionality will only work if you are logged into your account.