-(으)면 gives the moment and 알려주다 is letting someone know when it arrives: 우승자가 결정되면 알려드리겠습니다 (I'll tell you once the winner is decided), 준비되면 알려주세요! (let me know when you're ready), 결과 나오면 알려 줄게. Almost every line here is a promise or a request of exactly that shape, which makes it the ordinary way to say 'I'll keep you posted'.
Register: neutral; polite endings with strangers, plain with friends