Does searing a steak really seal in the juices?

No: searing does not seal in the juices. Although it is often said to lock in the moisture, searing actually loses more moisture than cooking to the same internal temperature without searing. Moisture in liquid and vapor form keeps escaping from a seared piece of meat, and the higher temperatures destroy more cells, releasing more liquid.

The idea has a name behind it. It was first put forward by the chemist Liebig around 1850, in his book Researches on the Chemistry of Food, and it was embraced by contemporary cooks and authors, including Escoffier. Experiments as early as the 1930s found that seared roasts lost the same amount of moisture or more.

So why sear? Because of flavor and contrast. To get a brown or black crust the meat surface must exceed 150 °C, and the browning creates desirable flavors through the Maillard reaction. The contrast in taste and texture between the crust and the interior also makes the food more interesting. The surface must also be free of water, which boils at around 100 °C.

If juiciness is what you want, the evidence points elsewhere: letting meat rest before serving makes its structure firmer and raises its water-holding capacity, so less liquid is lost during carving.