No, money does not magically appear in the account. Reimbursement means that you are withdrawing the funds that you put into the account. That's why you're putting money into the account. To take it back out and spend it on medical expenses.

The advantage is that you got a tax deduction for contributing the funds, and that you don't pay any tax to withdraw either.

Ideally, you should leave the money in the account and wait until old age to be reimbursed, that way you can invest the money in it.