First, It has a great values.As I stated, it is the first step to make clear about our territory. And the changes you did has a great impact to Japan. These small but impact changes make the world know about the reality of the japan's deed.
And naturally this will degrade the Japanese action to it and all around the world will know the truth.
Second, Japanisches sea is Japanese expression and it is our sea,
This sea is important to us because of the Dokdo problems. If everybody says this area to Japanes sea, then they will naturally argue that Dokdo is also their territory.
But as you know, Dokdo is our own territory and we don't want to lose it.
It has great supplies and Japan always want it. So, the expression east sea has great meaning to us and the first step to make clear about our territory.