A student of mine asked me about the usefulness of C-peptide in proinsulin, after some googling, I landed on this piece of information which seems to answer this question.
The answer is in the last line.
Proinsulin is synthesized as a single chain, 110 amino acid (aa) preproprecursor that contains a 24 aa signal sequence and an 86 aa proinsulin. Following removal of the signal peptide, the proinsulin peptide undergoes further proteolysis to generate mature insulin, a 51 aa disulfide-linked dimer that consists of a 30 aa B chain (aa 25 – 54) bound to a 21 aa A chain (aa 90 – 110). The 34 aa intervening peptide (aa 55 – 89) that connects the B and A chains is termed the C-peptide. Human proinsulin shares 84% and 80% aa sequence identity with rat and bovine proinsulin, respectively. Most of the sequence variation between species occurs in the region of the C-peptide. This peptide generates a structural conformation that allows for the correct formation of the intrachain disulfide bonds.