The most common reason to have a .CSV of your category or categories is to have an accurate count of your inventory while at an event or if you're selling your entire category or categories to another store.  

How to make a .CSV for a category:
Click on the Inventory tab > Products tab.  Locate and check the box next to the category you want to make a .CSV for.  Click the Export button.  Click the Notifications button at the top right of the page.  Click the Generated Reports tab on the black bar, to locate your .CSV that you created.