An essential part of any production team.
Quality control is so disregarded by SMEs that they have to wrap up their companies before they knew this is the actual core issue. I started my company with first tester(QC team) before i got the other production team members. Initially, it was a demand from my client and later i learned the importance of it. I learnt a good lesson by keeping the guy with me all through the projects i took in my company.
Whats it about ? Just that QC guy(s) are responsible if our client finds a bug(s) in production. They are the ones that let the client see the actual product ONLY after saying "YES" its a quality product.