The best analogy for the nucleolus in eukaryotic cells is a factory that makes tools that are used to build other resources. The primary function of the nucleolus is to combine and construct ribosomes. The primary function of ribosomes is to build proteins necessary for the cell.
In this analogy, one should consider that a civilization does not want hammers, saws and wrenches. Instead, human societies require finished goods such as houses and cars. Similarly, cells do not desire ribosomes; they desire proteins, which the ribosomes produce. In the tool factory analogy, a single tool may be used to build many different types of goods. Similarly, the cell’s nucleolus produces ribosomes, which are similar in structure but capable of producing an extremely wide variety of proteins.
Ribosomes are extremely numerous in an organism’s cells, and the nucleolus produces a large number of them to meet the protein needs of the cell. While many of the cell’s ribosomes are situated on the endoplasmic reticulum, many are also free to float through the cell’s cytoplasm. Occasionally, ribosomes clump together with other ribosomes. Some animals have up to 10 million ribosomes in their growing, functioning cells. Each time the cell divides, the nucleolus must produce double this number.