If you look at the four plants we take on Sukkot; you will see that one of the differences between them is the length of time they last until they dry out or decay.
The Etrog lasts the longest; it shrinks but can last for years without decomposing. Then, we have the Lulab, the palm branch, which takes a few months to dry (which is why, when the Matsot were baked at home, it was customary to use the dry branches of the Lulab, adding them to the oven where the Matsot were made). And then there are myrtle or hadasim, which can last a few weeks without decomposing. Of the four species, the most fragile and the one that decomposes the fastest is the ‘araba. Willow leaves last only a few days off their trees, hopefully, a week…
Our rabbis explain that at the end of Sukkot, the Heavenly Court issues the verdict on the rain that will fall this coming year (bachag niddonim ‘al hamayim). This is the main reason why, following a kabbalistic custom, it is customary to stay awake all night on Hoshanah Rabbah studying Tora).
“Rain” is the most critical element for our livelihood. And immediately after the end of Sukkot, the rainy season begins.
It is possible that the ‘araba also reminds us of our vulnerability and dependence on the rain that HaShem sends us. Furthermore, we internalize our fragility by hitting the ‘araba’s branches against the ground and seeing how the ‘araba is defoliating with each blow. Thus, we realize that we entirely depend on HaShem for sustenance and prosperity.