Deterrence doesn't only mean to discourage the perpetrator from recommitting the act, it means to deter the public at large getting doing so, for fear of punishment. So in the event of an accident caused by willful negligence, tyou would want to discourage others from being similarly negligent. For example, someone texting while driving who runs an intersection and causes a fatality. One reason to punish them is to impress upon others in your community to show care and awareness while driving.

I'm not endorsing this practice, just stating how I think deterrence is alleged to work.