You mean, besides the fact that they are all EU members???

Really, I don't get the idea that they are really "buffer" countries when they are part of the EU. But anyway ...

NATO currently has four multinational battlegroups in Estonia, Latvia, Lithuania and Poland. This represents many millions if not billions of Euros worth of investment in those economies,. Plus these small nations get access to big boy weapons systems and technologies that only the US and NATO can provide.

But for these Eastern European countries, NATO means being able to tell Russia to "f off". These small countries can hide behind Article 5, which they think makes them indestructible. They act as if NATO membership means they can treat Russia however they want; they do not need to treat Russia with respect, because they are in a gang that has bigger members than they.