While this coverage is secondary, in some cases it may help to cover your deductible if the rental car is damaged. But without having your own auto insurance to act as the primary coverage, relying on the insurance afforded by a credit card could leave you with significant gaps in coverage.
The coverage offered by the rental car company isn’t really insurance at all. What they offer you is a waiver. If you pay for this waiver, the rental company waives their right to pursue you or your insurance company for damages.”