We still use Zeplin beside Sketch because:
- Downloading single assets is very painful and time-wasting.
- The UX for inspecting is awful and slow. for every page, you should switch to inspect mode and it takes several seconds to be ready.
- As I mentioned here, Zeplin generates optimized assets automatically in different formats.
I should mention that Zeplin is very expensive in comparison to Sketch. 8 $ monthly for every developer and editor!