Answer 1

It is because in Java the main method is considered as the entry point for every program that you write.
Since it is an OOP language we use classes, objects and methods very often and only components like these make up the whole language.

Answer 2

you can write small code without methods if you want.
Methods are good for repeated actions, or for operate data in objects.

Answer 3

Sayhello
Is a method who write text hello??