The FBO should have the hull insured to a suitable level before you fly: Check this before you sign the rental agreement.
Renters' insurance in my case was to cover the cost of 'deductibles' or insurance excesses and is analogous to the collision damage waiver payment in car rental.
I had good reason to have taken out Renters' insurance many years ago when I totalled an Archer in the mountains: my Renters' insurance (cost $120) covered in full the cost of deductibles of $5K.
The hull loss was covered by the FBO/Owners hull insurance.
I had reason to be thankful as I had rented several times before in US and had never used Renters' Insurance previously.