In a practical way, nothing. But there are reasons other than practical results for doing things.
In a practical way, voting for a 3rd party candidate also achieves nothing in the US system (and despite having proportional representation even more votes, approaching 10%, were 'wasted' that way here too), in a practical way voting Trump in California achieves nothing since he won't win there or voting Biden in Wyoming achieves nothing since he won't win there; in a practical way this post and the one I'm replying to achieves nothing either. Doesn't mean that any of them wasn't worth doing though, even if they don't change anything, practically.
A formalised 'no confidence' option allows a gauge of those who are disengaged from the political process due to thinking the candidates- or electoral system- are awful but who would like to be engaged, and allows them to express their opinion without going into a generic 'spoiled' category that most will presume to be from people who cannot tie their shoelaces/ shirts button up down their back/ cannot successfully tick whatever box they really wanted to, rather than being a protest. That may not be much use practically, but in the more... figurative sense the purpose of elections is to allow people to express their feelings about who should lead them; and that certainly includes being able to express that none of the candidates are suitable, or that the process itself is broken. Which, for some reason, isn't a very appealing prospect for politicians.