Ophthalmologist Vs Optometrist

Ophthalmologist vs Optometrist?
If I see an ophthalmologist once a year for my eyes, do I need to see an optometrist once a year also? I wear glasses and contacts but last time I saw my ophthalmologist I have diabetes, my family dr makes me go to make sure my eyes are good . anyway last time i saw the eye medical doctor, he dilated my eyes, did a complete exam and wrote me a new prescription for glasses and contacts. why would i need an optometrist then ?? the point of my question is do I need vision insurance or should I opt out of it since my health insurance pays for the ophthalmology exam??