Steve's got this dichotomy stemming from his origin that doesn't seem to be explored in-depth but that continues to permeate his stories. It's this: he's both a super soldier and a super spy.
Project: Rebirth was a secret super-soldier experiment, largely foiled by an enemy spy. Since then, Steve has gone on public missions as a infantryman and on secret missions as a spy.
Sam the soldier serves as a great wing-man for Steve's public outings, while Natasha the spy helps him to navigate the shadows. Neither fills the super-soldier-spy role like he does, unlike Bucky.
Would be cool to see this dynamic explored further in the comics, with Steve somehow affected by the dual nature of his work.
Of course, this would mean comic Falcon would need another backstory retcon, since I don't think he's an airman in the comics.