Why using namespace std is essential?

Answer 1

It's not essential, in fact it's highly discouraged in the C++ community. That's because standard namespace has A LOT of things in it which means that if you include the namespace there can be possible name conflicts.

Answer 2

The practice of using namespace std was already discussed here:
https://www.sololearn.com/Discuss/294730/?ref=app

In my opinion, you shouldn't be using namespace std because of naming conflicts. And really, namespaces themselves were used to avoid naming conflicts.
Here are some examples that could show you when something like that could happen:
https://www.sololearn.com/Discuss/392399/?ref=app