I bought mine about 18 months ago after deliberating for quite a while following a few close passes on my commute. What finally persuaded me to get one were the 2 incidents within 3 days of each other. The first was riding home from work, I was aware of a noise behind me and glanced around, 2 vehicles behind, a black car and a builders-type pick up. The black turned off and I assumed the second one was full of his mates and they were shouting their goodbyes. How wrong can you be!! The next thing I was aware of was the passenger door beside me and the driver matching my speed, with the passengers face about 6" from mine, swearing and shouting that if I didn't get off the road he was going to kill me. Luckily I was turning off about 50 metres up the road, so I ignored him (and his mate in the open back, who was kneeling over the cab to add his 2p worth). I turned off they went straight on, but realising I they had lost me the driver reversed back and followed me down the road. I saw them coming and hopped off my bike and legged it across the road, thinking that I had at least the traffic coming in the other direction to make them keep their distance. Wrong again, the pick up swung across the road, narrowly missing a car coming in the other direction so that the passenger could lean across the driver and continue to shout ans swear at me. The driver only drove off when he realised I had my phone in my hand and had just asked the operator to put me through to the Police. After the had driven off, again swerving to avoid oncoming traffic, a motorcyclist and car stopped to check that I was okay, the bike said he was certain they were going to kill me.
The second incident was on my way to work, head down and struggling up hill I glanced up and saw a car heading towards me at reasonable speed, followed by white van man and another car. Luckily for me I was kept an eye on the traffic heading towards me as white van man decided to pull out and overtake, he even waved to me as I cycled on to the grass verge to avoid him, as I attempted to get off the verge the driver of the car behind the van decided it had to be clear if the van had overtaken and I ended up back on the grass.
In both instances the Police were either unable to take action (not enough mobile patrols in my area for the first incident) or just not interested (no witnesses to the second). At least with a camera the Police would have had video evidence to find the drivers.
That is why I decided to get a camera, if that has made me a nob, so be it, the camera records on a loop, so unless something drastic happens I don't even look at it, but if I end up dead in a ditch at least someone will hopefully look at the video and my family will might get justice.