The question is a deceptively simple for reasons upon which I will elaborate. Firstly, people use umbrellas for the purpose of remaining dry or 'not wet', specifically when their surrounding area is being rained on, pelted with wetness as it were, but whence this desire for retaining dryness when one's environment is wet wins this human urge to ostracize oneself from the natural cycle or rather to ostracize nature from one's own life. Is the clue not in the question? Is it not the same with modern man's wish to spend his life in cities; Homo-urbanus, remove from the tumult of the organic world and yet what is this world and what is rain? Rain wets which providing plants and animals with nutriment, filling up their cells with cytoplasm and their veins with blood to transport nutrients allowing them to stay alive. Rain wets, providing the ground with moisture, moisture that seeps down to the sedimentary layers of soil and without which no repositories of drinkable water would be present for the urban human to hydrate himself. Urban man is desiccated but relies upon the natural cycle to replenish him. Yet he pretends that this ancient arrangement is beneath him, as when he removes his domicile from the wet ground as when he removed his body from the rain. The umbrella is the symbol of man shutting nature, of woman shielding herself from the life-giving sky. Rain wets and it arose that cracking ground creating landslides, waterfalls, floods. It reforms the terrain, allowing new flora and fauna to make their inroads in the service of our world and yet from this modern man and woman shield themselves. Why? Why to keep their clothes dry, to keep their electronic items secure. For you see rain wets and kills technology and that is the bane of homo-sapiens. What sapiens is there erecting the atavist of your life and society on a foundation that is enemy to the water. Who's anathematized cloud and air and sea and sky for water is in all those things and then what is an umbrella. An umbrella is an illusion, a self-delusion that you can keep the wet out of your life that though you breathe and eat and walk on things have used with it, you can control the meaning of water over your existence. What could be more intolerably vain then the child wishing to control his parent, except the child believing that he can and is. It is the delusion of created over creator. Of art over artificer but is that truly so mad? What is the artist if not the channel for the divine, the whisper of muses in the song of demons wherein lies the purpose of the artists of artistry whose essences its product the work of art and water is the artist, animal creation the art of human civilization that crowning jewel, the final stroke the paintbrush. Why do people use umbrellas in modern society? Because they are life.