I don't get it too....back in the days people used to hold on paying RPRF so that they know PR was coming but it has changed now....I think once you clear the eligibility check they ask for RPRF fees...then most of the files get stuck on Security hence the delay....RPRF request is another useless indicator. Some have gotten it early on and spent months waiting and some have gotten it later on and got PPR within weeks and some have gotten it in the middle. IRCC seems to change when they send the RPRF request randomly or maybe it differs from VO to VO.
I don't understand why people don't pay it upfront... even the IRCC website recommends paying it upfront to ensure faster processing.