Not everyone in the covered group wears glasses, so the cost to the insurance company is less than $300 * N. Offering that is a nice benefit to the people who do need glasses.

In the meantime, Zenni is sending me good-looking properly made glasses for as little as $30/pair shipped. (Single vision, polycarb, titanium memory alloy frames. Arrived 4 days after I ordered them last week.) So much of the $300 is really going to line the pockets of Luxottica https://en.wikipedia.org/wiki/Luxottica

Yes, though it's still weird as an `insurance': most adults already know whether they ever need glasses or not.