The deposit is a security measure for the local rental company. Starting when you pick up the vehicle, the deposit will be blocked off on your credit card for the entire rental period.
The amount of the deposit is usually the amount of the stated deductible. In the case of damage or an accident, it covers the costs (e.g. theft of the rental car or damage to the bodywork).
For the deposit, it makes no difference whether you have booked the rental car with or without a deductible. Nevertheless, the deposit is always paid in full. Even if you have booked an insurance package with a refundable deductible, the local rental company, in the case of damage or theft, will retain the deposit. Your provider can then later refund the deposit once the claim has been settled.
The insurance package without a deductible (in other words, a refundable deductible) is refunded in the case of damage or theft.