What BCAAs actually are
Branched-chain amino acids (leucine, isoleucine, valine) — three of the essential amino acids found in complete protein sources, isolated and sold separately, often flavored for sipping during workouts.
Where the original popularity came from
Early research suggested BCAAs alone could stimulate muscle protein synthesis — but more complete research since has shown that a full spectrum of amino acids (from whole protein or whey) does this more effectively than BCAAs in isolation.
Why they're largely redundant with adequate protein
Anyone already meeting their daily protein target through food or a complete protein supplement is already getting more than enough BCAAs as part of that protein — additional isolated BCAAs provide little extra benefit in that context.
Where they might still have a niche use
Training in a fasted state for an extended period, where a low-calorie way to reduce muscle breakdown is useful, is one of the few scenarios where BCAAs offer a distinct advantage over simply eating enough protein around training.