Blake says that Bruce came to see him when he was in the orphanage. We know he left the orphanage when he was 16. We also know that Bruce operated as Batman for around a year up until the end of TDK. This means that Bruce went to the orphanage at some point around TDK or a little after. Blake can't be a cop at that age so the character needs to grow up. Well how do you allow him to grow up and fullfill what is required in TDKR? Time gap inbetween TDK and TDKR. Eight year gap conveniently makes Blake 22-24. A believable age for someone to be a cop. I didn't need to work on the script or be in the planning to figure that out. Without the gap Blake wouldn't be old enough to do what the plot requires.