This happens because Dell ships many systems with enabled. By default, pressing F1–F12 alone triggers the action key (volume, brightness, etc.), and you need Fn + F# to get standard F1–F12 behavior.
Unlike standard modifier keys (Ctrl, Alt, Shift), the Fn key doesn't send a command on its own. Instead, it works as a dual-action trigger . It toggles the secondary function of keys (usually labeled with small blue or white icons). fn button on dell keyboard
Save this post for the next time your brightness keys seem to “stop working.” 😉 This happens because Dell ships many systems with enabled