Because in certain countries the ISP has to respect the legal regulations and so the DNS server provided/defined by each provider will block/redirect certain web sites. These could be torrent trackers, subtitle distribution sites, political and/or religious sites and so on... In some parts of the world alternate DNS servers allow people to access all sites :-)
>>NKosma+(OP)
Not to mention that some ISPs redirect users to pages full of ads when a domain doesn't exist or use DNS to MITM users and inject ads into pages.