I thought I understood sprints

I watched the qualifications highlights and saw what happened with Perez. So, I thought the qualifications were for the start position in the sprint. Obviously not since he started front row. And now that the sprint is over, and I see the lineup for the race, perez is starting 15th. I thought the sprint was used to determine the start positions for the race. How were the start positions determined for the sprint? What is the purpose of the sprint if it doesn't determine the start positions for the race?