interface design - Why are slices in Sketch Important?

I understand that this a noob question; I am about to import my Sketch design into Zeplin.io and I am being told that I should have slices created for my assets. I have created symbols for my icons that have a rounded pixel value for spacing. Why would I need to now create slices for the assets in my project?

I am told that I should create slices for an easy integration for development; but this seems a little redundant.