what is document cover generally used for?

Answer 1

Hi. A title and protection of the main document. On a resume, it is a letter addressed to the company or person to whom you direct the resume.

Answer 2

It's used to tell you what the document is about (title), the purpose, who wrote it, when it was published, who published it, who printed it, when it was printed, copyright information etc. etc.

But I think it's mostly there so the next page won't get dirty he he i.e. protect the content and sometimes make the document look more interesting than it actually is.