Wow . . . . ok.
When a device is recording it does so in chunks. The size of those chunks on some dashcams is configurable. You can have lots of small chunks, say 1 chunk every 30 seconds, or a smaller number of large chunks of say 5 mins each.
How you configure the device will depend on your preference. If you are reviewing an accident including the minutes before, during and after the incident, would you prefer that to be all one continuous stream of video, like a YouTube clip? Or would you prefer it to be 50 separate small bits of video that have to be viewed separately or put together using some video software (=hassle for some)?
Why is this an issue?
Because if you have chosen to record longer chunks (say 2-3 mins each) then when you stop your car and switch off the engine, the camera still needs to complete writing that chunk to the SD card. Without a battery, you would be switching off the power when you switched the engine off (if hard wired or if your power socket isn’t always on). That would result in that last chunk of video being lost and not recorded.
That could be the vital footage of the accident just at the point the collision resulted in your engine switching off.
There may also be delay/lag in the entire recording process so it may affect more than one chunk of video.
So the battery power is important for ensuring video writes are completed.
Aside from that, having a battery means you can remove the dashcam from the car (if it’s not integrated) and use it as a digi cam yourself. Maybe using it to walk around the accident scene, film the damage to cars and so on.
Bruce is correct that any device can be opened up by a competent “tinkerer” and with a bit of Googling you could identify and order a replacement battery, whip out a soldering iron and fit it. For most people though this is undesirable, not least because doing so would invalidate any warranty left on the device.
I stick to my personal preferences/recommendations which are that a dashcam should be tiny, discrete and integrated and therefore should be fitted with capacitors not rechargeable batteries. That will guarantee longevity and mean you don’t need to keep removing the device every time you leave the car.
Ideally I would have bought 2 of the Mobius Action Cams (about £50 each). One I would integrate in the mirror housing and fit with capacitors. The other I would leave as is, with a rechargeable battery and keep aside to use to film the accident scene by hand.