The celebrations of Simhat Beit HaSho’eva would continue all night in anticipation of the drawing of the water at dawn and the libation of the water upon the altar. The Sages explained why this mitzva was performed specifically on the festival of Sukkot.

And for what reason did the Torah say: Pour a libation of water upon the altar in the Temple on the festival of Sukkot? The Holy One, blessed be He, said: Pour a libation of water before Me on the festival so that the rains of the year, which begin to fall after Sukkot, will be blessed for you. (Rosh HaShana 16a)