1. No versioning needed. For designers versioning has been a huge pain, some git like tools have popped up but with Figma none is needed. Changes made by multiple parties are always in sync since they are made live in the same doc. Plus always on versioning.
2. All in one tool. Currently designers use sketch(create designs), Invision (prototype and get feedback, and Zeplin (hand off to dev). Thats 3 different places to manage, no source of truth. With Figma its all in one package.
3. Cheap. Free for teams of 2 designers, viewers are always free. You done need to pay for the above tools I mentioned.
4. Strong foundation for plugins and community. They way the figma team have built the product allows for rich plugins and community.