Searing isn't about “sealing in the juices”. It doesn't. That sizzling sound you hear is the sound of meat juices evaporating in a hot pan.
But searing beef is important for two reasons.
Firstly, food safety. Searing destroys any pathogens that might be on the surface of your meat. At 74°C they are destroyed instantly.
And, secondly, you sear for taste.
Between 140°C and 165°C the Maillard reaction occurs. This creates an avalanche of new flavour compounds – which enhance the flavour.
It's absolutely essential to dry your meat thoroughly before you sear it. If it's wet the pan uses all its heat to evaporate the moisture, rather than heat the meat. And you need that heat to get a good crust on the meat. It should take a minute or so to sear per side.
Give the pan a chance to heat up before you put your meat in. That'll take you at least three minutes on the highest heat.
And watch out for the smoke.