Why we need the documentation comment in Java?

Answer 1

You can generate doc files (html) from the Javadoc comments.
You wouldn't want to use an api without Javadocs, which allows you to read the documentation for every class without opening it from an external source. The IDEs link these doc files and show them on a mouse over or something right from the source code you're writing.
Very useful.

Answer 2

Archana

So the next coder/developer does not have to waste time, figuring out what the code does.

Comments should have enough detail to understand how the code functions.