When you rent a car, you take on full responsibility for any loss or damage suffered by the vehicle whilst it is in your care.
A damage liability waiver partially reduces - or "waives" - your liability for such loss or damage.
It is an agreement between you and the owner of the hired vehicle that your liability is partially waived in this way, rather than an insurance policy in the strict sense of the term (though most people will reasonably see it as such).
The easy answer to the question of whether or not you need damage liability waiver, therefore, is that is not essential - but you would be landed with full responsibility for any loss or damage that happens to the vehicle, and this could potentially be as much as the replacement value (a lot of money, in other words).
Most rental companies, therefore, will want to protect their investment in the vehicle by insisting that you have an adequate damage liability waiver or its equivalent.
Many will include the cost of providing the waiver in the quoted cost of renting the vehicle.
Where this is the case, however, it is important not to be misled into thinking that this totally absolves you from any financial responsibility for damage to the car.
There are important exclusions usually attached to such agreements.
Damage to the windows or glass, the tyres and wheels, and the roof and underside of the vehicle, for example, are all areas most often excluded, yet these happen to be areas that are among the most vulnerable to damage.
Since they are excluded from the damage liability waiver, this means that you could end up paying for the whole repair bill in the event of damage.
It is also important to remember that the damage liability waiver is almost always only a partial waiver.
The agreement includes an excess amount excluded from the waiver that remains the responsibility of the hirer of the vehicle.
The amount of excess is likely to vary from rental company to rental company and from one country to another - but is still a significant amount.
In Europe as a whole, it might be several hundred pounds, but in the UK it is frequently £600 and in Australia, for example, it can be as much as £1,500.
Just like the excess on a regular insurance contract, the damage liability waiver excess is the first part of any claim for loss or damage.
To take the UK example of £600, this means that if repairs for any damage cost, say £500, the hirer of the vehicle will end up paying the whole of the bill.
Given the size of the hirer's remaining liability under the damage liability waiver, therefore, it is also possible to insure either a part or the whole of the excess, thus reducing the risk.
This separate cover for the excess is likely to be sold by the rental company itself, but there are a number of specialist car hire insurers that offer more competitively priced products.
Standalone excess insurance from an independent provider, therefore, is certainly worth considering.