Pages is such a useful app, and nicely priced too. It has the advantage of connecting up with the iCloud which means I can work on documents on my Mac, iPhone or iPad. It all seems to work seamlessly. However, not everyone I know has Pages. Many people use Word so if I want to share with them I need to convert it. By the way, I use Word a lot too.
The other reason I might want it in Word or as a PDF is if I need to upload it to a website. I know students have to upload assignments and in many instances they prefer Word or PDF, especially if it is to check for plagiarism.
Here is a little video I made to show you how to export from Pages to Word or PDF.