There was logic explained at the time.
It was noted that few inner suburban passengers at Kings Cross left the station, they almost all went into the Underground, to either West End or City. Sending the trains to Moorgate allowed direct travel to The City, and a far easier cross-platform interchange to the Victoria Line at Highbury for the West End.
It also relieved congestion at Kings Cross, both access to the Undergronnd, which was inadequate then, and the rail side congestion at the station throat, which in the days of loco-hauled services was considerable as well. It likewise allowed outer-suburban passengers for The City to make cross-platform interchange at Finsbury Park. I seem to recall the calculated numbers per hour of each of these flows were in Modern Railways at the time.