The customer is telling you over the phone while reviewing the draft: "Make this bigger, change that color to yellow." It's hard to keep track of what to make bigger and which blue to change to yellow. Now, with this website, you can upload your design and the client or supervisor can leave notes with suggested revisions on the parts that need to be changed. These notes will become to-do items and can be worked on one by one, making the proofreading process more efficient.
Update: 12/9/2020 - The service seems to have ended. QAQ