Why must I purchase a life- insurance ?

A rental vehicle was reserved by me with Venture in California. The price was not $9.99/ night as a result of weekend offer. When I checked in at the counter the agent told me that I needed to both supply evidence of FULL COVERAGE insurance or acquire their insurance (which might improve my daily price from $9.99 to $40)!! They would not I’d like to rent a car basically didn’t do both then