Webflow is more like a visual development tool. You are not as flexible and fast with designing your page in Sketch first, but your result will be a real page :) It is super intuitive and I highly recommend Webflow to designers which want to create a real webpage without coding. So it makes sense to use Sketch first for a screendesign, because you are faster and flexible, you can try out more ideas. When you have finished your design you should go to Webflow and start rebuilding your design, like a developer would to it with your designs.
Webflow is more like a visual development tool. You are not as flexible and fast with designing your page in Sketch first, but your result will be a real page :) It is super intuitive and I highly recommend Webflow to designers which want to create a real webpage without coding. So it makes sense to use Sketch first for a screendesign, because you are faster and flexible, you can try out more ideas. When you have finished your design you should go to Webflow and start rebuilding your design, like a developer would to it with your designs.