One favicon used to be enough — it isn't anymore
For a long time, dropping a single favicon.ico in your site's root folder was the whole job. That stopped being true once phones, tablets, and progressive web apps entered the picture — a browser tab icon, an iOS home screen bookmark, and an Android app shortcut are three different contexts that each expect their own specifically-sized image, and a site missing any of them either shows a blurry, wrongly-scaled version of your logo or falls back to something generic, like a screenshot of your page instead of your actual icon.
This is why sites that only ever set up the classic favicon.ico often look slightly broken specifically on mobile — the desktop tab looks fine, but the home screen bookmark doesn't.
What each specific size is actually for
The 16×16 and 32×32 PNG sizes cover browser tabs and bookmark bars at different display densities. The 180×180 apple-touch-icon exists specifically because iOS doesn't use favicon.ico for home screen bookmarks at all — without that exact file at that exact size, iOS silently falls back to taking a screenshot of your page and using that as the bookmark icon instead of your logo, which looks unmistakably broken. The 192×192 and 512×512 sizes are for Android's Chrome and any progressive web app install prompt, referenced through a site.webmanifest file that tells the OS which icon to use at which size.
None of these sizes are arbitrary — each maps to a specific real-world display context, which is why generating the full set at once, rather than guessing which ones you'll need, is the safer default.
The manifest file people forget to actually check
The site.webmanifest file is easy to treat as a black box you just drop in and forget, but it's worth actually opening after generating it — it typically ships with placeholder values for your app's name, theme color, and background color, and those show up visibly if a visitor installs your site as a home screen app or PWA. A generic placeholder name showing up on someone's home screen instead of your actual site name is a small but noticeable miss that's easy to fix in two minutes if you remember to check.
Where these files actually go
Generating the files is only half the job — they need to be referenced correctly to work. Most go in your site's public or root directory, with explicit <link> tags in your HTML's <head> pointing to each one (favicon.ico is often auto-detected by browsers if it sits at the root, but the others generally need an explicit tag). Skipping the link tags is a common reason a freshly generated icon set still doesn't show up correctly after deployment — the files exist, but nothing on the page points to them.