This is likely a side effect of the ending being different from what was originally planned.
However, us developing along the path that the Reapers desire is still somewhat useful to them. If I lock you in a room with 12 doors, each with a puzzle to unlock, and unkown prizes or perils behind them, but instead, afer 8 hours I open the door I want you to go through, you wont find your own way, you will find my way.
If we had not found the Relays, and developed in the exact way the Reapers desired, who is to say we may not have invented something ourselves, something different, or even better than the relays. That is an amount of chaos and unpredictability that would make something like a harvesting cycle needlessly more difficult for the Reapers.
Without the relays guiding our path, we could be anywhere in the Galaxy(which as we know, is huge). With the Relays, all civilizations are within reach of the Relays, and basically plotted on the map for the reapers to harvest. Hell, even with this path it still takes hundereds of years for them to route out and find everyone for harvest.
Yeah left unchecked and outside of the guided path, we might not even develop murderous super AI, and boy would that put the Reapers in a pickle