There are a few good reasons.
1. You never know what act of stupidity you will catch on it. There are entire tv shows now dedicated to dash cam footage. Accidents, road rage, you name it.
2. If, as Juan noted, you have some dillhole effing with you on the road, you have something to show Johnny Law, if need be.
3. Speaking of Johnny Law, I live close to a small community known for having a few B team members with a penchant for pulling people over for silly things. One headlight brighter than the other, driving out of the wheel tracks, not stopping before the stop sign, etc,etc. Having a dash cam may help keep one out of trouble if the newbie is kind of reaching on cause to pull you over. None of us do well in court when it comes to our word against the local LEO.
Anyway, I'm just surprised they aren't an option on modern vehicles straight from the manufacturer.
I hope everyone's driving goes safe and smoothly, but I also know a fair number of people that in hindsight wished they had a dashcam.