Why do interoffice envelopes have holes in them?

By drilling holes, the envelopes are able to get past some US Postal codes for regulations. If the envelope is totally closed, it fits into these codes and violates the law. i.e. Should be managed and/or regulated by the US Postal Service. The other common answers also make sense. The holes drilled in the envelope allows you to see if the envelope contains anything, without having to untie the string to look inside.