Sear a steak hard first and you seal in the juices, locking the moisture inside so the meat stays tender and the crust holds it all in like a wall. Every cooking show, every barbecue uncle, every recipe says it. The sizzle is the sound of the juices being sealed.
Searing does not seal anything, and you can prove it at home: a seared steak still hisses and leaks juice all over the board when you cut it, because there is no waterproof seal. The idea was popularised in the 1800s by the chemist Justus von Liebig and repeated as gospel ever since, but food scientists, including Harold McGee and the team at Serious Eats, have tested it directly and found seared meat loses as much or even slightly more moisture than unseared meat, because the high heat drives water out of the surface. What searing actually does is fantastic, just different: the intense heat triggers the Maillard reaction, a cascade of browning that creates hundreds of new flavour and aroma compounds and a delicious crust. That is the real reason to sear, and it is reason enough. So sear your steak, absolutely. Just for the flavour and the crust, not for a moisture seal that does not exist.