Here's why:

* Ribosomes are the protein synthesis factories of the cell. They are composed of rRNA and proteins.

* The nucleolus is a specialized region within the nucleus. It is the site of rRNA synthesis and the assembly of ribosomes.

* rRNA genes are located in the nucleolus. These genes are transcribed by RNA polymerase I, which produces precursor rRNA molecules.

* These precursor molecules are then processed and assembled with ribosomal proteins to form the functional ribosomal subunits.

So, while the nucleolus is involved in other processes, its primary function is to produce rRNA, the essential component of ribosomes.