We are told in Bereishis 1:12 that on the third day of creation:
וַתּוֹצֵ֨א הָאָ֜רֶץ דֶּ֠שֶׁא עֵ֣שֶׂב מַזְרִ֤יעַ זֶ֙רַע֙ לְמִינֵ֔הוּ וְעֵ֧ץ עֹֽשֶׂה־פְּרִ֛י אֲשֶׁ֥ר זַרְעוֹ־ב֖וֹ לְמִינֵ֑הוּ וַיַּ֥רְא אֱלֹקים כִּי־טֽוֹב
The earth brought forth vegetation: seed-bearing plants of every kind, and trees of every kind bearing fruit with the seed in it. And G-d saw that this was good.
Fast forward to Bereishis 2:5
וְכֹ֣ל ׀ שִׂ֣יחַ הַשָּׂדֶ֗ה טֶ֚רֶם יִֽהְיֶ֣ה בָאָ֔רֶץ וְכׇל־עֵ֥שֶׂב הַשָּׂדֶ֖ה טֶ֣רֶם יִצְמָ֑ח כִּי֩ לֹ֨א הִמְטִ֜יר ה' אֱלֹקים֙ עַל־הָאָ֔רֶץ וְאָדָ֣ם אַ֔יִן לַֽעֲבֹ֖ד אֶת־הָֽאֲדָמָֽה
When no shrub of the field was yet on earth and no grasses of the field had yet sprouted, because G-d had not sent rain upon the earth and there were no human beings to till the soil
Rashi famously explains this to mean that despite the vegetation being created on the third day, it did not sprout from the earth until the sixth day, when Adam realised the need for rain and prayed accordingly.
However, in subsequent posukim which relates the creation of Adam, the verse writes:
וְאֵ֖ד יַֽעֲלֶ֣ה מִן־הָאָ֑רֶץ וְהִשְׁקָ֖ה אֶֽת־כׇּל־פְּנֵ֥י הָֽאֲדָמָֽה: וַיִּ֩יצֶר֩ יְהֹוָ֨ה אֱלֹהִ֜ים אֶת־הָֽאָדָ֗ם עָפָר֙ מִן־הָ֣אֲדָמָ֔ה וַיִּפַּ֥ח בְּאַפָּ֖יו נִשְׁמַ֣ת חַיִּ֑ים וַיְהִ֥י הָֽאָדָ֖ם לְנֶ֥פֶשׁ חַיָּֽה
but a flow/mist would well up from the ground and water the whole surface of the earth — G-d formed Adam from the soil’s humus, blowing into his nostrils the breath of life: Adam became a living being.
Clearly, there was a type of rain/natural irrigation that was used before Adam was created so why couldn’t that same rain have been used to water the vegetation?
The Netziv asks why it specifically had to be an אד הארץ מן יעלה and answers that it wasn’t exactly a type of גשם (rain) and instead Hashem used an אד until Adam came. However, this doesn’t yet explain why the אד that came down didn’t help to make everything grow?