Technically, he’d been in the process of inventing it for a while. October 22, 1879 is simply the day on which he first successfully tested a practical incandescent electric light bulb.
Reliable electric light is one of those inventions that changes the world. Sure, we aleady had candles and gaslight, but electric light was both brighter and more reliable than its predecessors. Edison changed the world with it. He changed the hours of our work and leisure, and the way we think about both.
And being Thomas Edison, he went on to invent still more stuff, many of which also changed the world, but none so much as the humble light bulb.