Cytoplasmic streaming
Streaming is often seen in large plant and animal cells. Cytoplasm moves inside the cells. The flow is driven by forces in the cytoskeleton. The function of the flow is probably to speed up transport of molecules and organelles around the cell. Cytoplasmic streaming is usually found in  large cells, larger than about 0.1 mm. In smaller cells diffusion of molecules is rapid, but it slows  as the cell size increases. Larger cells need flow to move nutrients inside them rapidly enough for efficient working.