I didn't actually understand what is use of operator overloading?

Answer 1

Operator overloading is kind of operator customization... You can write custom methods for using operator with custom data: if you define a custom class 'Thing', compilator don't know how to handle:

Thing instance1;
Thing instance2;
Thing instance3 = instance1 + instance2;

( this is the same as the @Dean implicit example )

If you have overloaded operator +, compilater will know what to do for handle it ^^

Answer 2

It's syntactic sugar; rather than writing vec1.add(vec2) you get to use the more intuitive vec1+vec2