The event I want to attend has a «money-back guarantee». Why is it worth taking out additional ticket insurance anyway?

The «money-back guarantee» applies if the organisers cancel the event. The «money-back guarantee» ensures that you as a ticket buyer will be refunded the ticket price within one week of the cancellation.

You take out the ticket insurance for yourself. If you are unable to attend the event because you are ill, for example, you can submit the certificate to the insurance company and get the ticket price refunded.