Words have power. Words build things that aren't there, flesh out people who don't exist, and make visible that which only the mind can see. They have moved the most stolid of hearts to laughter and to tears. They have fomented wars and invited peace. Words are even so powerful as to prompt feelings in us even in the absence of context. Mention "stink" and already our noses wrinkle up in disgust; mention "love" and see how our skin flushes with warmth.
Words give mass to concepts that men have suffered under, fought and died for. Words like slavery and freedom, words like revolt and injustice.
Words are timeless. Your words speak to your contemporary audience. But they also reach backward and forward in time. Backward, as you are joining that great conversation with every other being who thought to put finger to keyboard, pen to paper, quill to papyrus, chisel to tablet. And forward, because fixed words carry onward for later generations to share in the conversation.
Spoken words serve their purpose well enough, but are fleeting. They're gone the moment they're said.
Written words have permanence.
Writing them makes you immortal.