These days it is important to utilize any means possible to ensure that when you mail out a payment (check money order etc) there is better then a 50/50 chance that the payment you are sending will get there using a secured envelope. That is why we use security envelopes all the time when making a payment. Putting in a regular unsecured envelope doesn't guarantee the same security for sending sensitive information. Hence why it's important when sending deligate information or payment you use a secured envelope.
We use them as offering envelopes. In each box some of the envelopes were already sealed therefore could not be used. Although we were able to use more than we lost the lost was large enough that we won't order any more for fear of the same thing happening.