Most of high traffic sites use managed DNS like dnsmadeeasy / ultradns, etc.

My website has 60,000 page views a day and currently we have DNS records managed on GoDaddy (which is free) and the records point to my server in Amazon AWS.

Why do these big sites use managed DNS?

How is it better than the free DNS provided by domain name providers?

Should I go for it and what would be its advantages?

Does managed DNS also help in greater availability of the website because sometimes ISPs too have issues with DNS resolving and sometimes my site becomes unaccessible or doesnt load properly on some ISPs / locations?