The best reason has got to be for health care. If you have an accident and need emergency assistance the cost for your treatment (and even a rescue if you're in a remote place). I heard of one British guy who fell ill in the US and ended up incurring over $500k of costs. I guess some places won't treat you unless you're insured - although again that might only apply in places like the US. So even if you have millions in the bank, that might not be good enough for the healthcare provider and might lead to delays while they check you can fund the treatment. And if you have millions in the bank why skimp on travel insurance?

Certainly, many travel companies, tours etc won't let you even start a trip unless you have insurance.

Insurance can also cover you with personal liability - again all fine if you're Bill Gates and can write a cheque. For everyone else that's a potential life saver.