The most useful feature is being able to preview your work before you share it with clients or get paid by them as they can see exactly how what their ads would look like! You have no need to use another software when using dizpliy since all features are built-in.
Sometimes I find myself in trouble because sometimes there's a glitch which makes me unable to save my changes (specially if I do something wrong). So be very careful. It has helped me create multiple versions of ads/ad spots without having to use other tools such as Photoshop.