rain man

וְכֹ֣ל׀ שִׂ֣יחַ הַשָּׂדֶ֗ה טֶ֚רֶם יִֽהְיֶ֣ה בָאָ֔רֶץ וְכׇל־עֵ֥שֶׂב הַשָּׂדֶ֖ה טֶ֣רֶם יִצְמָ֑ח כִּי֩ לֹ֨א הִמְטִ֜יר ה׳ אלקים עַל־הָאָ֔רֶץ וְאָדָ֣ם אַ֔יִן לַֽעֲבֹ֖ד אֶת־הָֽאֲדָמָֽה (Braishis 2:5).

Rashi comments:

לפי שאדם אין לעבוד ואין מכיר בטובתן של גשמים. וכשבא אדם וידע שצורך הם לעולם, התפלל עליהם וירדו, וצמחו האילנות והדשאים

Hashem only brings rain after the creation of man, when there is someone who can appreciate the rain and pray for it (see Maharal in Gur Aryeh).

The gemara (Yevamos 63a) tells us that אתם אתם קרויין אדם ואין העובדי כוכבים קרויין אדם. The word "adam" refers not to all mankind, but specifically to the Jewish people. Sefas Emes writes in light of this Chazal that when Rashi tells us that G-d delayed the creation of rain until there was an "adam" to ask for it, he means specifically the prayers of Klal Yisrael (I assume he means that Klal Yisrael existed at least in potential within Adam haRishon).

Rain is needed for all humanity, not just for the Jewish people. And there are plenty of people and nations who pray to G-d for rain when they need it. Our prayers, however, are different. Our need for rain is because "im ain kemach, ain Torah," without food to eat and our material needs being fulfilled, we would never be able to devote ourselves to any higher spiritual pursuit. Our Shimini Atzeres, where we ask for rain, goes hand in hand with a Simchas Torah.