Tu Quoque is when you dismiss someone's criticism by saying that the criticizer has the same flaw. Everyone thinks it's a fallacy, but it's often not. The examples that people give for it are usually ones where the criticizer acknowledges that they have the same flaw (eg. me talking about how the purpose of life is to be a perfect person while I spend hours every day rereading my own content for fun and browsing memes on reddit). In these cases it's a fallacy. But if I were to say all that without acknowledging my hypocrisy, that would indeed be unfair and people would have the right to dismiss my arguments. It makes no sense to say that my opponents have to deal with my arguments validly while I don't. I don't get a free pass just because I shot first.

Another way to look at it is this: if I refute an opponent's position, but my opponent shows that my own position is also incompatible with the principle I used to do it, then both of us have been refuted. I do not win the argument.