Why do I need health insurance?

Insurance of any sort, including health insurance, is a means of transferring the financial risk of an adverse occurrence, from the insured to an insurance company. By that definition, health insurance is necessary if one wishes to effect the transfer of financial liability for certain medical, hospital and arguably, prescription expenses from (an) individual/family to an insurer.

All of that said, health insurance is not a necessary in the sense that food or water is. One can make a conscious choice to be uninsured and to be individually responsible for one's own medical expenses.