It just means that I'm conceding the possibility that I'm wrong. I'm not sure it "adds" anything particularly, except for scrupulous accuracy.
I see it as part of the same argument as solipsism. If you're strictly applying logic, then you have to concede that all the information you have about the world is garnered through your senses and so you have to concede the very slight possibility that your senses are completely wrong and that what you're experiencing as reality actually isn't reality at all. You could be, as I've said before, in the Matrix, or you could be a brain in a jar with electrodes attached, or a computer simulation of a brain programmed to believe it's human, or so on. I think that you absolutely have to concede these possibilities and uncountable others like them, because you can't know with absolute certainty that they're not true.
But what does a philosophy based on those concessions tell us about the real world? Nothing. What use are they interacting with the real world? None. Is there any reason to doubt that the real world outside your brain exists? No. So, while you logically have to concede that things outside your brain might not actually exist, it's not something it's worth spending any actual thought on, unless you're doing so for the sake of an interesting conversation.
Is it useful? No. Is it something you have to concede if you're being 100% scrupulously accurate? Yes, I think so. Agnosticism, at least as it applies to me, is pretty much the same. In fact, were we to draw a Venn diagram of my solipsism and agnosticism, then there would definitely be areas of overlap.