A favicon is the small icon displayed in a browser tab, bookmark bar, or search result next to your website's name — typically just 16 to 32 pixels, among the smallest applications a logo is ever used at.
Because of that tiny size, a full logo — especially a wordmark or a detailed combination mark — usually needs to be simplified into just its core icon or an initial to remain legible as a favicon.
A brand system built with a simplified icon or lettermark variant from the start makes this adaptation easy; a logo designed only as a single, detailed image often requires additional simplification work later.