This is required by law. Since there is some risk involved, you should put funds into a spending account only for those expenses you feel certain will be incurred. If you leave Rensselaer for any reason, any money remaining in an account can still be used for expenses incurred while you were employed at Rensselaer. To be reimbursed for medical expenses incurred after you leave Rensselaer, you would have to continue making contributions to your account. However, this would be on a post-tax basis.
More detailed information is contained in the Summary Plan Description.