Saint Patrick was just a missionary. He probably did little to improve the lives of the Irish and instead came to the island to fill people up with mysticism. He never drove out the snakes because there never were any to begin with.
Many of us celebrate this day because we are all or part Irish. Some of us support Irish nationalism and the Irish cause, with the symbol of the Starry Plough.
For many people this holiday is just an excuse to get drunk. "Burp!"