While it is said that an image speaks more than a thousand words, there are a few reasons why adding a thorough item description is very important when selling online.
Adding a description makes your listing appear to search engines. Sellers should keep in mind that search results (both in Yaga and Google) are displayed based on the keywords added to product description. As more and more people search using specific keywords such as "coat", "dress", "nike sneakers", then adding this kind of information is key to improving product discovery and therefore sales.
It helps prevent questions and saves you time. Even though an item may seem attractive and suitable from the photo, a buyer also needs information about the size, condition etc. If that kind of info cannot be found in the description you may have to start answering questions one by one (plus may loose potential customers who just drop off because of insufficient information). Also, if there are any specialities to the item (it's not true to size, oversized cut) it's always better to include this to the description instead of dealing with the buyer's disappointment later.
You help the buyer to determine the price & quality relationship. It's always good to mention the brand of the item since it can help the buyer to identify whether the price is attractive enough. Also, many people search specifically for brands like Nike, adidas, zara, Calvin Klein etc.
In conclusion each item should include at least the following information: brand, item, size, colour and condition. For example: Yellow ZARA skirt, size M. In great condition - worn just once!