To enable automatic product descriptions please go to Catalog > Settings > General and check the Category descriptions on product page checkbox located inside the Categories settings section.
Once you enable this option the category descriptions will show up in the product description. Each assigned category description in separate paragraph. Starting from top category through all the subcategories until the bottom subcategory is reached.
This way you can create unique descriptions automatically. You can still add your own description to each product (it will show up along with the automatic description).