I guess I don't see any functional difference. I suppose to send some kind of message, but it's not like there's anyone listening.
But by your logic no one should ever vote because their individual vote means next to nothing on the grander scale, and yet people do vote and ultimately their votes decide things. If you don't vote, you're sending the message that you can be safely ignored. If you do vote, even for a candidate who in theory stands no chance of winning you're sending the message that you do vote and you will vote against whomever it is you don't like. If you choose not to vote, you only contribute to an idea that one or more of your demographics doesn't vote and therefore can be disregarded to allow more room to pander to another.
tl;dr if you really think no one's listening, then you stay home and don't vote
Due to the electoral college many people's votes are effectively meaningless in presidential elections. There is really no point in my voting for president, for example, because I live in Massachusetts, a state which will reliably vote for the Democrat in every circumstance. The same is true for someone who lives in a state like, say, Alabama, which will vote for the Republican in every circumstance.
If you cast a blank ballot, no vote is counted. But they don't know that you, a specific individual, cast a blank ballot. They can look at the overall numbers and see that ten thousand people cast blank ballots, and yes, collectively that might send some kind of message, but the numbers would have to be significant before anyone would start paying attention. I'd kind of like to see it, actually, but I'm not holding my breath.
If anyone was listening we never would have gotten into this situation in the first place, with the two worst candidates in living memory. Like I said, I'm writing myself in.