The security deposit covers repairs for damage caused during a rental or any breach of contract (mileage package exceeded, missing fuel, delay, etc.). As such, the security deposit may be used to cover an insurance excess rate.
The insurance excess rate is the amount up to which the renter is liable in the event of an incident. Beyond this, the insurance company will cover the rest of the expenses if the damage is covered by the guarantees and the user is not in breach of the insurance terms of the policy they are subscribed to.
For more information on the security deposit click here.