TARGIT Reports are just one type of documents that you can create with the TARGIT Designer client. The other main type of documents is the Dashboard type, but, for instance, you can also create Slideshow documents.
The main purpose of a TARGIT Report would often be to produce a layout and an output suitable for being printed on A4 or Letter formats. The output, rather than being physically printed, often stops at the PDF file.
Furthermore, to a large extend, TARGIT Reports are used with Scheduled Jobs to automate distribution of essential data. With Scheduled Jobs you can set up a TARGIT Report (or any other document type) to be send out automatically, e.g. once a week, as a PDF file attached to an email to a number of recipients.