What was the need of symbol primitive type to JavaScript language?

ES6 story...

By the way, Symbol primitive value type was the last value type added to the JavaScript beside null, undefined, boolean, number, string, object.

• What problem as a whole this new type is trying to solve?
• What problem you as a developer faced and tried to solve via Symbol?

Please share... I'm gonna have a full list of answers from every one... from their point of view.