Because it solves crimes.

I was on a jury where the police were able to track down a hit and run driver who killed a teenager within about 45 minutes due to red light cameras.

There are lots of things that we could do to solve crimes but we don't because of the negative effects of doing them.

Torture is a good example.

Torture is not a good example. If you torture someone enough they will tell you whatever you want them to tell you in order to stop it.

Mike Baker talks about why it was pointless for the CIA to torture someone and why when they train people how to handle being capture, they tell them to tell the truth with a little information redacted.

Solving crimes is a minor bonus for society. If they had prevented the driver from killing the teen, then that would have been a much more compelling argument. But people running after a fatal crash doesn't make situation much worse, and the incentive to avoid fatal crashes doesn't change massively if you know for sure that you will be caught rather than being almost entirely sure.

In most of the comments about solving crime, its never about prevention but rather retribution.