As an attorney I send out many letters to clients some of which are multiple page opinion/advice letters often with copies of other documents attached for the client's easy reference. Sending such documents all crunched up in a #10 envelope (as some attorneys send documents to me) is infuriating and frustrating for the recipient. A flat manilla 9x12 is an inexpensive way to show "class" to a client.
I have a large accounting business & have a large number of clients during the income tax season esp. I've used these envelopes for many years in our business.