Every man has an opinion on shoulders, lapels, break. Almost none have one on the drop. the quiet piece of maths doing more to shape a suit's silhouette than any of the details we obsess over out loud. It's the difference between a suit that fits and a suit that fits you. 

Once you understand it, you stop guessing at what "slim" or "regular" actually means on the rack, and start choosing a fit with intent.