The short version: vertical (9:16) is built for phones and dominates Instagram, TikTok and Facebook Reels. Horizontal (16:9) is built for screens and still leads on YouTube, websites and, mostly, LinkedIn. The honest answer for most businesses is that they need both, which is why every job I shoot gets planned around the platforms it's actually going to. Framing a shot for both formats from the start is far cheaper than reshooting later.