It was developed back in the days when Southerners thought it useful to make sure that the Democrat candidate was a white man ~ women, blacks and Republicans were easily kept out of the primary/runoff system.
I think that if we didn't have a primary runoff the Republican candidate in my house district would have not been strong enough to have beaten the 14-term congressman in my district. The front runner in the primary who had just over 32% in the primary lost the runoff to the second place finisher who had 30%. The process of campaigning for the runoff exposed some of the front runner's flaws especially in the candidate debates even though he was a much better looking candidate than the second place finisher who eventually won the runoff.