One of the main reasons we still use Zeplin besides Sketch is that Zeplin generates optimized assets in different formats (Webp, png, jpeg, svg). It is a very handful feature and saves a lot of time.
You can do this in Sketch too, in the Export section in the Inspector.
If you have a selection of filetypes that you export to often, then you can also create and save your own Preset Group, so you can apply it anywhere with just one click.
In real products, there are some common workflows that are impossible without Zeplin:
I have a library of more than 1000 symbols of icons. With Zeplin I only need to make them exportable with one click in Sketch. Then when I use them in any document, Zeplin generates all assets automatically.
without Zeplin, this common workflow takes infinite time. Also, it is very common to forget to make an icon exportable. with Zeplin you don’t need to worry about this.
If you use an icon symbol in another symbol, it is impossible to make it exportable without detaching symbols!!!
@Xhaust mention this issue here: