Not an essential tool in your armoury but, as well as the reasons above, a rear cam could also show some dickhead hooning down the empty lane just as you enter a roundabout in the correct lane.
I'm still waiting to hear from our insurance about the idiot in the van that cut me up on the roundabout - despite a witness who would testify to the guy speeding down an empty lane.
A rear cam would show that he entered this lane (for example) 10 cars back, average 12ft per car = 120ft. Covering this distance in X seconds - therefore he was travelling way faster than his claimed 25mph & possibly way, way faster than the posted 30mph speed limit.
Years ago, I had a mini drive into the back of me, shunting me forward so I almost his the car in front. This left the mini enough space to pull around me & suicidally pull into fast traffic to get away. My front cam only caught the back of the car in front of me. The rear cam caught nothing since I didn't have one. Made a few marks on my plastic bumper & bent all my brackets for my cycle carrier. Even a cheap camera would've got them bang to rights.