It would allow for many blueprint tools to help with development. For example you can have a test map firing range. Do you feel your weapon fires too slow? Just press “+” and the firerate default value increases. It saves you time from stopping the play session, going to the blueprint and editing the value manually, the starting the play session again to test. This would only work in the editor. I am aware the above example can be accomplished with save games, but it would be cleaner this way.
Another way would be just printing the new weapon values as a debug string on screen. This way you can adjust your values and once you are happy just use the lastones printed as defaults.
This way you would only have to edit them once (you are satisfied).