I've got the Black Vue 590W with rear channel... this is the kind of footage you can get (incredibly this quality was only 77MB which is nothing nowadays), this was funny as the guy on my right (that you can't see) was reving and showing off and asked for a race with his missus... this is what happened lol. Just a bit of fun like, but it's one of a couple l downloaded off the BlackVue to my phone which is really handy using wifi
However, yes I believe a rear camera is fantastic for anyone who shunts you and I love mine. Not cheap though so £100 won't buy you BlackVue... mine records at 4k front and 1080P on the rear. Loads out there but I feel they add that extrata "idiots hits you, you have evidence"... but could ALWAYS be used against you don't forget that!
Personally I wouldn't buy just a front one... I think rear is vital, but that's just me! As for parking, mine triggers when stationary and records ONLY when Gforce hit it i.e. think there's a 2 second delay post hit detection and then it records so could also help with the muppets in car parks etc!!! If I get their reg, always a bonus!