Q:
Why do we wish our fellow Jews a כתיבה וחתימה טובה or a גמר טוב? Does that really help the other person?
A:
It depends. When you say to somebody כתיבה וחתימה טובה will it help him? It depends. It depends with how much sincerity you are saying it. If you’re just saying it מצות אנשים מלומדה, without thinking, then it’s worthless. Mamash worthless. But of you’re thinking when you say it, and when he walks away you say it a second time – yes, when he walks away you should say it a second time – then it means something. You hear what I’m telling you? When he walks away, say it over again slowly. כתיבה וחתימה טובה. And even better – add your own words. “רבונו של עולם, please give this man a year of happiness and parnasa and שלום בית.” He doesn’t hear you. But now you mean it.
When you do it that way, הקדוש ברוך הוא listens. Hashem listens to tzadikim, and you’re a tzadik if you pray for him in that way. ותן כבוד השם לעמך. Hashem listens. Certainly He listens. When tzadikim give you a bracha, it means something. Of course it means something. Tzadikim can help by giving a bracha.
And same thing with שבת שלום. When you say it the first time it doesn’t mean a thing. So when he walks away, repeat it. Say “שבת שלום. Have a wonderful Shabbos. Enjoy the chulent. Hashem should give you a good nap.” Ohhh, now you’re talking! That’s something; that’s an achievement!
(Rav Avigdor Miller)