Buying collision casco insurance is optional (as opposed to mandatory third party liability insurance). Collision insurance covers self-inflicted damage to one's own vehicle. Some companies include a waiver of reduction in benefits in case of gross negligence in their policy, others sell this kind of protection separately.
Some insurance companies call a combination of partial casco (only comprehensive) and collision insurance full casco insurance. Full casco insurance only makes sense for newer cars up to an age of 4 to 5 years.
For leased vehicles, collision insurance is mandatory.