Why does some financial mail come in perforated envelopes rather than normal, sealed envelopes? Is it because they contain sensitive information, and normal envelope seals can be carefully opened and shut using steam, allowing unscrupulous people to read their contents, while these types of envelopes can't be as easily opened and re-closed? If so, why doesn't all mail with sensitive information come this way, and how well does this actually protect you?
In addition to a possible security benefit, these types of envelopes save paper over traditional envelopes. The "fold and tear" envelopes, after you unfold them, are a single sheet of paper, printed on both sides. A traditional envelope has separate sheets of paper for the envelope and the payload insert.
I don't know for sure which is more expensive to send, but I could see the "fold and tear" envelopes as being less expensive than traditional envelopes. A single sheet is printed, glued, and folded, and it is ready to send. With traditional envelopes, the payload is printed and folded, but then must be stuffed into a preformed, windowed envelope in the correct orientation so that the address shows through the window. The traditional envelope has more material, more steps to complete, and more risk of defects.
The downside of the "fold and tear" envelopes is their inflexibility: you only get one sheet. Traditional envelopes can hold several sheets and a return envelope. The "fold and tear" works great for IRS tax forms like the W2 or 1099, which are small, standard size forms that fit on one page. However, for something like bank statements or utility bills, which are usually multiple pages, often include extra advertisements, and require a return envelope, the current versions of the "fold and tear" system wouldn't work.
In practice, I don't really see it as much of a security benefit. If someone wants to steal the info, they'll just open it, read, and dispose. You'll never know you got it, you'll request a new copy, and it will be assumed it was lost in the mail. I think the reason behind the popularity of this type of envelope is the cost advantage.
If you search on line you can find these "fold and tear" envelopes either in a continuous sheet or as individual sheets. There are even special machines that will fold and seal them automatically.
One advantage for pay stubs is that it can easily print the information on the bottom as a check or just "advice".
One advantage for some standard tax forms is that they come pre-printed with all the required info giving the explanations for each box on form. Some even come with perforations so that the forms can be attached to the federal and state returns. The tax and accounting software can be configured to work with a specific paper product making it easy to prepare all the mailings in one batch.
It easy to spot these envelopes in the mail becasue of their form factor. In situations where they send tax forms in a regular envelope they generally put the phrase "important tax documents inside". Of course the easy to spot form factor makes them less secure if somebody wants to steal your identity.
A long time ago I even saw a variation where tore off the top as a receipt and the rest of the document was the envelope for you to send in your payment. The envelope had the correct address to return the check.