I think the point is that the Rainham service was a last minute tack-on after the Bermondsey diveunder was already under construction. Its purely to get trains from the core out of the way because they can't get them through Windmill Bridge and East Croydon as things stand. That's the thinking that drove the decision to run 700s to Rainham. Do not be misled into thinking that the service is actually there out of some altruistic desire on the part of GTR to benefit the good people along the southern bank of the Thames. It also did SE a favour, allowing them to get rid of the DOO metro work beyond Gravesend (the Gillinghams), which presumably makes it easier for TFL to run it when they inevitably take it over.