Answer
Proteins can form a wide range of polymers performing many, many functions in the cell due to the four levels of structure which they have. Primary sequence has $20^{N}$ possible sequences for a polypeptide N amino acids long, which is a very large number for a normal protein of 100 amino acids long or longer. Then, the next three levels of folding provide a wide arrange of shapes to build tools to do many jobs in the cell.
Work Step by Step
The key here is to see that there are two sources of great variety in proteins: the primary sequence and the folding. This leads to an enormous array of potential shapes and, thus, functions for the cell.
