Well designed, assets right next to the design and fast. Variety of options to upload different asset's.

Ability to see how a site will look and feel before going live.

Zeplin works really well with Sketch. The integration is seamless, updating screens and artboards is a easy, and the Zeplin markup and notation tools are intuitive and helpful. Organizing artboards within Zeplin is very straight forward and it is easy to keep large projects organized. Building a style guide that can actually be implemented in code is really easy. It's not perfect but it works really well and version control is really good.