The vue set global api method in vuejs can be used to set a property of a reactive object. In other words it is a way to add a property to an object in the data of a Vue constructor instance, and have the view update. The thing about this is that many frameworks have a method like this, because sometimes you can not just simply add a property to an object, sometimes some additional things need to happen on top of that.
For a quick example of vue set, I put something together that makes use of a property in the data object but only if it is there. So in the example there is a root property in the data option object that is also an object, this is impotent because there is no way to add root properties to the data object with vue set. However vue set can be used to add properties to an object that is nested in the data object.
When I use the vue set method to set the value of the mess property of the u object in the data object the message bar is displayed as expected. If I where to just add the property directly the message will not display, because it is not reactive. This is the whole reason why there is the vue set method in the global api of vuejs, it is a way to add properties to a reactive object after the fact.