Why One Image Isn't Enough for a Favicon
A single favicon image file doesn't cover every context it needs to work in — browser tabs, bookmarks, mobile home-screen shortcuts, and OS taskbars all request different pixel dimensions, and a single large image scaled down naively can look blurry or lose important detail at the smallest sizes (16x16 is genuinely tiny — fine details and thin lines simply don't survive that scale-down).
Generating multiple explicit sizes (16x16, 32x32, 48x48, and often larger for mobile/PWA contexts) lets each context use a purpose-fit version rather than the browser stretching or shrinking one image awkwardly at render time. At 16x16 specifically, a logo with fine detail or small text becomes an illegible smudge — which is why effective favicons often simplify to a bold, simple mark (an initial, a simplified icon) rather than a full detailed logo, something worth considering before uploading a source image that's actually a full wordmark logo rather than a standalone icon.
Browsers also cache favicons aggressively, which is why a newly generated favicon sometimes doesn't visibly update immediately — a hard refresh or cache clear is often needed to see a freshly deployed favicon change.