Automatically extracting attachments from Gmail can be important for reasons where you need to process the attached files periodically with a CRON job. Also it can be useful for automatically archiving important attachments. Below is a simple proof-of-concept plain PHP code, devoid of any object-oriented features that extracts attachments from your Gmail account. It uses PHPs imap extension to access the inbox.
His simple script opens an IMAP connection to the Gmail server and loops through your current emails (max of 16) and tries to find any that have attachments. It then extracts this part of the message, base64_decodes it, and saves the set of them to the local file system. You can download the source here.