Why does grass make pollen?

Plants have pollen because they need to sexually reproduce. In the same way that humans make eggs and sperm, in a plant you have a stigma which leads down to the ovary and the plant's equivalent of eggs are, and pollen is like the sperm. Pollen floats around in the air and is carried there by insects as well. They land on the stigma and grow invasively down into the stigma with a pollen tube. The egg is then fertilised and you get a seed forming. The plant spreads that seed so it can make new versions of itself.