But as mentioned before, they were defacto NATO anyway. Now this just makes it obvious to everyone that they are in. Issue for Finland though is that they can have forces stationed in their country - but if war ever to break out, those forces are within line of sight of Russian artillery and rockets from within the border. And the Russians may be less inclined to be cautious in dealing with them as they are in dealing with Ukraine. Instead, it could possibly flatten the country rather quickly and say "screw the civilians".
All this will do is force Russia not only to expand its military in terms of manpower and equipment - but to also put pressure on Finland economically.