Just to be clear:
You MUST have Israeli 3P liability insurance. It's the law. You will have it - and pay for it - regardless of the way you book.
The only difference between various agents, otas etc is if they include it in the quote they give you, or it gets added to your total at checkout.
CDW can be covered by your CC. If it is not, the rental agency will make you pay for at least the lowest level of coverage.
Agents can help for: Getting you a better deal/rate, getting a rate that only charges the insurance charge on shabbos, getting a reservation even when its zeroed out, getting a second/third driver for free, reducing the airport fee, getting additional KM on a short reservation, assistance with the paperwork if you ever have damage that needs to get claimed from your CC