What I mean is, how can a retired General help Trump politically or with relationships with Congress and the GOP?
Personal assessment (and I may be very wrong): he possibly can't much, but that doesn't matter. Trump wants people who'll be loyal to him above all. He doesn't really care about the party of political clout. That's because he's fundamentally misunderstanding the structure of government and his own role as president.
He basically thinks his job is like that of a CEO. Except he also misunderstands what a CEO really does. The Trump Organisation is more like an oversized family business than like an actual corporation, where even CEOs aren't free to act like dictators.
He doesn't get on some very basic level, that being president is not like being a mob boss where everyone follows your orders. And that's why he'll pick people not for their suitability but based on how likely he thinks they are to agree to place horse heads into beds ...
See his recent beef with Sessions who - albeit a horrible person and someone I've arguably never agreed on a single policy position with - is basically qualified for his job. But he recused himself from the Russia investigation (correctly so!) and thus showed disloyalty. Hence the recent public dressing down.