Sometimes people forget to write descriptive subjects when they send mail. Sometimes they just drop some files there and send a message with a blank body. And if the attached files have names like 1.doc, your chances to find that mail later are almost null. Unfortunately, Gmail doesn't have an annotation feature that would let you insert some comments in a mail you've just received. To make sure you'll find that messages, you could send a reply to yourself that contains a small description of the attachments.
It's a small compensation until Gmail has an option to search the contents of attachments.