Simple. If the driver killed the guy's dog and broke his leg, and in his grief and anger he accused the driver of speeding, the police would naturally look in to this... although it could not be easily proved anyway. Video footage would put it to rest straightaway and no doubt save a lot of hassle. Always better to have it than not. There are many more examples of course... and in no incident would anyone ever say, oh I wish I'd never recorded that... unless it WAS their fault lol!
That wasn't the situation though - my question was about the actual situation that he described, where nothing actually happened