Due to baby Kal-El's super powers, Earth clothes didn't last long on him. So, Martha Kent used Kryptonian blankets to make a costume for him. If you can see the second image of the attached answer, the baby costume also had a cape (not just the original Superman costume).
So, I can assume that it was either Jonathan Kent's or Martha Kent's idea. Why did Jonathan and Martha Kent (who weren't good at physics, I think) attach a cape to the costume? What was its purpose?