Officially, they are pushed forward to allow people in dictatorships to break laws restricting their freedom of information and/or expression.
Because, the problem with the law is, that you need some people to make it, and wherever people are involved, they try to get their beliefs into the law. As such, laws, even in so-called "democracies", are not always based on common sense, real-world requirements or scientific research (and then, scientists are people, too, putting all their beliefs into their research). Moreso in dictatorships, where freedom of expression ends whenever the ruler thinks that his power is endangered.
And you need guards to check that everyone abides by all the laws. Some say that the NSA is such a guard. These people have beliefs, too (like Edward Snowden). But then, who guards the guards? Who supervises that they don't break the law (like the CIA did, as the torture report revealed - yet, not one torturer will be sued or anything. Is this lawful?).
Then, there's another problem about laws. It's time. What is lawful now, may be unlawful next week. Furthermore, in many countries, things done now are seen as having been unlawful as of next week. Especially with freedom of expression, you may be put into a concentration camp by the next President Of The Reich about opinions you expressed five years ago. Some people who think that future rulers may oppose their views, will only express them in anonymity. That's what anonymity networks are thought for.
But: Like everything you create that allows someone to break an "unlawful law", whatever that is, it can be used to break a "lawful law", whatever that is.
And: If some ruler can identify or shut out people who break "lawful laws", the next ruler can also identify or shut out people who break "unlawful laws".