“Endgame” opens right before Thanos snaps his fingers and wipes out half of all living things in the universe. We see Hawkeye lose his entire family, unaware of what just occurred.
“This scene was supposed to be in ‘Avengers: Infinity War’ after Thanos snapped his fingers,” said codirector Joe Russo on the film’s commentary. “We thought it might be interesting to cut to a verite scene between Hawkeye and his family.”
The Russos thought it was too disorienting and busy to have at the end of “Infinity War” when they were already cutting between people disappearing in Wakanda and in space. The start of “Endgame” wound up being the right fit for the scene to remind viewers what happened at the end of “Infinity War” and how you felt.