Taobao currently only serves mainland Chinese customers and majority of sellers are only able to communicate in Chinese. A Taobao agent helps you communicate and purchase items at Taobao on behalf of you.
Payment options like MasterCard and Visa are not widely accepted in China. If it is, your credit card company will charge you 3-5% of the total. This is why choosing to buy through an agent is recommended. Furthermore, it saves you the hassle of having to complain to sellers if an issue arises because an agent takes care of everything for you.
By purchasing through an agent, consolidation of products bought from different sellers is possible. This will save you a ton of money and time.