What could this be used for?
What does its existence mean?
Sodium-39 created in Japan.
https://physics.aps.org/articles/v15/177
It has 2.5x as many neutrons as protons and exists at the location below on the nuclide chart.
It should not exist. 
Discussion
Huge advancements in the nuclear physics and energy fields! It can help us understand nuclear reactions and make nuclear fusion reactions less challenging to contain. If this were possible than it would mean that we would be able to produce a MASSIVE amount of clean energy! This is a big maybe, though! 👀🤷♂️