At the moment, the criminal law—in the West, at least—is based on the idea that the criminal exercised a choice: no choice, no criminal. The British government, though, is seeking to change the law in order to lock up people with personality disorders that are thought to make them likely to commit crimes, before any crime is committed.
Such disorders are serious pathologies. But the National DNA Database being built up by the British government (which includes material from many innocent people), would already allow the identification of those with milder predispositions to anger and violence. How soon before those people are subject to special surveillance? And if the state chose to carry out such surveillance, recognising that the people in question may pose particular risks merely because of their biology, it could hardly then argue that they were wholly responsible for any crime that they did go on to commit.
Nor is it only the criminal law where free will matters. Markets also depend on the idea that personal choice is free choice. Mostly, that is not a problem. Even if choice is guided by unconscious instinct, that instinct will usually have been honed by natural selection to do the right thing. But not always. Fatty, sugary foods subvert evolved instincts, as do addictive drugs such as nicotine, alcohol and cocaine. Pornography does as well. Liberals say that individuals should be free to consume these, or not. Erode free will, and you erode that argument.
In fact, you begin to erode all freedom. Without a belief in free will, an ideology of freedom is bizarre. Though it will not happen quickly, shrinking the space in which free will can operate could have some uncomfortable repercussions.