By looking through the source document you can determine the subject, the purpose of the document, and the size of the translation project.
You need to know if you will be given enough time to do a quality job. If you know you can't meet the deadline, then there is little point in asking for more information.
The target text
You need to know if the translation is for publication (any kind of distribution) or for information only (internal use). Or maybe only a summary or outline of the source text is needed in order to decide whether or not the entire text needs to be translated at all.
You also need to know about the intended readership of the translation and how the target text will be distributed and used.