what's the difference / benefits between using a managed DNS such as zoneedit, dyn.com and Amazon Route 53 and a standard DNS manager provided by domain registrants such as goDaddy.
for the tasks I need like add/modify A, CNAME, MX records, do I need a managed DNS?
They're all "managed DNS" in the meaning of "someone else runs the infrastructure for you".
What you need to look at are your requirements, such as:
Most registrars just allow you basic A, CNAME and MX records and not much else, just via a web interface.
If that's enough, as it is for most small sites (like blogs etc) then great! Otherwise if you need an API, or want to give different A records to different regions, then you need to shop around.
