Quite often, when you rent a car online they offer basic insurance, which will cover theft and etc. However, in most cases it will not cover windows, tire punctures and etc.
From my experience, there is no point taking insurance online from a rental company if you want it fully insured (windows and tires included). Also quite often you might want scratches to be included as well (as we found out in our search of waterfall) especially if you are planning to drive in rural areas, so make sure to ask for that as quite often that’s excluded.
Even if you buy insurance in the rental company when picking up a car it doesn’t mean it will cover everything with $0 excess. If they ask you to leave a deposit, that means it will be the minimum amount you are paying if you have an accident, scratching and etc. In most cases I guess it is better than paying full price for incident, which can be very costly for a new car.
Also, if you traveling frequently and renting cars it might be beneficial to book annual car insurance online prior to your travel. It might not include scratches, but will cover all other bases and will cost less.
https://www.insurance4carhire.com/
https://www.icarhireinsurance.com/
In addition to that, you can buy insurance to cover your insurance excess. An insurance for an insurance! For example: https://www.insurance4carhire.com or on Travel supermarket