Cops often demand unlawful searches, order dispersal, or deletion of photos/video when they have no legal basis to do so. If you don't comply with their order, they'll arrest you for obstruction, failing to follow a lawful order, resisting arrest, wiretapping or any of a number of 'contempt of cop' charges.
Dispersal orders aren't valid unless there is a threat to public safety. Each area has it's own specific rules, but I haven't read one that I thought was unreasonable. Cops simply use the law as a threat since in court it will be the officers word against yours regarding whether the conditions were met -- unless there is video. That is why video is so hated. They can't lie as easily.
My encounter this year was a speed trap. There is a budget short fall, and the traffic cops had been sent out to help - I mean 'ensure traffice safety'. They'd been setting up seed traps and after each citation they'd pull the next car passing by over to issue another. I knew they were doing it, so I'd been driving with video rolling showing my speedometer and the road out the front window since that's about the only defense from revenue focused traffic enforcement. I was pulled over for 51 in a 40 zone, despite never going over 40. That is one of the simplest reasons everyone needs to video each encounter with cops. It's a partial defense against for profit law enforcement. (so Yes, I have video and it did happen)