In projects where mobile is the primary way an audience will view a site or app (and this number will probably only ever increase; one of my clients has 80% mobile views for example) – it often makes sense to start the cascade 'mobile-first' rather desktop first as webflow currently works – it helps designers, developers and stakeholders keep focussed about their audience, the content, the effectiveness of the UI decisions and importantly, generally just 'think' more from a mobile-orientated point of view (+ arguably some performance gains depending on how the cascade and code is setup).

If a majority of your audience use mobiles – I can't think of any advantages of starting desktop-first. I find webflow's approach encourages mobile views to become basically stacked versions of the desktop view – and while agree that is essentially what most mobile sites are these days (and is often the best solution), I think the approach generally discourages more creative solutions to optimising the experience for small screens.

(Discussed at length in the forums, often with much argument!)