The main point is that lithium is hard to get for European companies. Sodium is abundant in saltwater, which is good for European companies and good for a more massive use of batteries (electric cars and home storage).
If they can demonstrate that this battery technology is viable for large grid storage uses, there is a potentially enormous market for it in Germany. The problem is that solar pv makes most sense in Southern Germany, where it's sunnier, but a lot of the power is required further north, so there is a requirement for major new powerlines to be built[1]. Also, there is wind power generated in the North Sea that is needed further south.
Providing massive, (relatively) cheap battery backup to these intermittent power sources could allow Germany to accelerate the transition away from coal, which they've been forced to use more of due to decommissioning nuclear plants. If the power companies are willing to invest heavily in infrastructure for renewables, it bodes well for them investing further in batteries to improve grid stability and prices.
The problem in Germany is transporting power from wind to the south, and not transporting power from PV. And Germany didn't use more energy from coal since the shut down of nuclear. (renewables replaced nuclear)
Cheap batteries would allow every country to move to renewables.
Sodium-Ion batteries have a few advantages - they can be completely discharged without taking damage. This is a huge deal for cargo (remember the cargo plane that caught fire because of lithium batteries a while ago?)
