It all depends on how you drive/use the vehicle, how long you want parking mode to run and how sensitive your vehicle is to discharging the vehicle's 12-volt battery.
We have three vehicles and two of them do not have a dashcam battery pack installed. Two of the vehicles are driven to locations where they will be parked for at most 9 hours. A properly charged vehicle battery is capable of powering the dashcam in those two vehicles while parked for that amount of time. Both of those vehicles have the dashcam's installed with hardwiring devices with low voltage cutoff capabilities.
I have another vehicle that might be parked at a remote location for 12 to 16 hours. Even a fully charged vehicle battery will be questionable of powering parking mode for that long without setting any voltage limiters/cutoff to a low voltage (anything below 12.2 is a low voltage in my definition). I'd rather have a dashcam battery pack powering that camera's parking mode recordings to make sure all of the parking time can be recorded without depleting the vehicle battery's charge level.
Hybrid vehicles will often come with a super small 12-volt battery which is responsible for powering a variety of the 12-volt features of the vehicle (a group 51R battery in our 2006 Toyota Highlander Hybrid). If that small battery's charge level is depleted, you won't be able to "start" the engine/hybrid systems and it requires a "jump" of the 12-volt battery system just to start it even though the hybrid system's battery pack may be fully charged. A dashcam battery pack might make more sense in a vehicle with a small battery or a vehicle that is hypersensitive to having its battery's charge level reduced by a dashcam.
Just like with most things in life, a dashcam battery pack is not a one size fits all type of device. There are times when you can get away without using one and then there are times it helps protect your vehicle's drivability by not discharging the vehicle's battery to power a dashcam for parking mode recordings.
On the pricing, yes dashcam battery packs are expensive! It's hard to say buy a budget XYZ dashcam with super wonderful features for $120 to $250 and then if you end up needing a dashcam battery pack to power it (for reasons stated above) spend two to three times the price of the dashcam to buy the battery pack.