I don't particularly need Zeplin as such, even though we do use it at work. But something that…
Allow others to view designs without having to install software
Allow others to comment on designs
Download designs, and download exports.
… is a necessary part of a design process. If you're a distributed company, even more so.
Copying CSS is nice to have, but far, far from the importance of the above. The generated CSS doesn't tend to be that good anyway.
With Serif commitment to Affinity products being non-subscription, this could potentially make business sense as a subscription based supplement, if developed by you.
Zeplin integration provided by Serif doesn't make sense to me. In this case, exposing an API and making the integration a task of the third party makes more sense.
Edit: This is basically what's keeping me from dropping Sketch. Dropping Sketch would be a great relief to me. The way Sketch does components might be a little bit more intuitive perhaps (although it's not that great either to be honest), but other than that, AD is definitely better in every respect.