According to the Halachah, to the Jewish Law, what is the first thing we should do, once the Yom Kippur fast is over? Following logic, the most appropriate answer would be "eat." But that is not the answer. Custom has it that one should separate or cut off a little branch that will form part of the sukkah for the Succot celebration, which is always held five days later.
Why? Probably to connect the spiritual to the material, two kingdoms that in Jewish civilization always go hand in hand and are practically impossible to separate. To pay attention to the spiritual and forget the concrete is to be perceptually lame. And the same applies to the inverse situation. Succoth comes by way of balancing our existence, and to remind us in a whisper that spiritual fortitude must also unfold within the marvelous fragility of matter.
Simchat Torah, the crowning glory of the Chagei Tishrei, combines both universes when the Sefer Torah is carried around in a dance, clearly indicating how the respective domains are to be sustained. Quite an art form!