OpenDNS are independant DNS servers to resolve your records with. If you browse to
www.google.com a dns server translates this to an IP to send your request too.
Often providers have buggy and unstable DNS servers, or because of governement rules block certain websites from being resolved. OpenDNS does not.
If your ISP's DNS server works fine there is no reason to switch to OpenDNS.
Sometimes OpenDNS is also usefull if your ISP only permits DNS requests from their own network, and if you have a laptop and sometimes work remote but don't want to change your DNS settings everytime.
Cheers