They're unmanned and ambient, yet are clearly a provocation and give China an information advantage over where it would be without the balloons, and in a geopolitical sense it asserts Chinese ascendency. At the same time, it's hard for the US or other powers to figure out an appropriate response. Very similar to Russian/NK/Chinese/Israeli/American state-sponsored hacking groups--it continually forces the adversary to ask "where do we draw a line, and what consequences do we give for crossing it?"
