Scoop, it is confusing. It is common for physicians and hospitals to contact the next of kin and ask them for consent for necessary medical procedures. Most of the time this process works. However 1) if there is controversy it can be difficult to determine next of kin (which parent? which sibling? significiant other?) 2) depending on state law authorization for some types of care (like administration of research medications or HIV testing) may only be given by the patient or someone legally authorized to consent for care, which usually means a legal guardian or a health care proxy 3) Physicians may be reluctant to share information about sensitive topics such as reproductive health, drug use, mental health issues, etc. with family members if there is not a designated proxy. 4) Chedva’s already mentioned the really wrenching decisions about life support etc.
People get sick and physicians/hospitals work with families all the time without a proxy. However, you never know if you will be the one caught in one of the situations above, struggling to do your best for a loved one without the clear legal authority to act.