Think of all the moving parts in your engine. Specifically the most important to have pressure, the point where the rods swing on the crankshaft, and where the crankshaft swings where it's held in the block. What oil pressure does, is create a buffer between the crank and the bearings. Remember that the crank is just slightly smaller than the bearings it sits in. By having pressure in the oil that is in that small space, it keeps the parts from touching (hopefully). Just oil being there isn't enough in places like that. It needs to keep the crank centered in the passage. If oil pressure drops too low, the crank will contact the bearings. Shortly after, bearing failure. After that, the crank is ruined. Then funny noises, and eventual engine seize up. That's why oil pressure is so important.