Having the Fremantle character allows the director to explain events and Lee's strategy to the movie audience in the guise of explanatory conversation between Longstreet/Berenger and the slightly-out-of-touch Brit. It's a pretty common theatrical device.
Having said that, the portrayal of the man is teeth-clenchingly stereotypical. No doubt the actor, James Lancaster, was taking direction, but as a Brit I am always relieved when the movie narrative moves on.
Fremantle did not wear uniform at the battle; even so, the clobber that the movie costume department have him rigged up in is way, way off the mark.
Chances are that Fremantle had purchased his commission and advancement, and it is undeniable that the system led to incompetents achieving rank that they could never have achieved by ability alone. By 1863, though, the system was on its last legs, having been seen to fail disastrously in the Crimea. It would be abolished in 1871, but it's true to say that even though commission by purchase was no more, social status and family connections counted for a lot in the selection of British officers for many decades to come.