Adjustable splitters with locking are typically found on premium helmets, and they solve a specific annoyance: strap creep. With standard sliding buckles, the straps below your ears can slowly shift over weeks of riding, requiring periodic readjustment. Locking splitters let you position the straps exactly where you want them, then lock them in place so nothing moves.
This feature matters most if you ride frequently or find yourself constantly fiddling with your straps before or during rides. It's also helpful if your head shape doesn't align well with fixed strap positions, since you can dial in that proper V-shape below your earlobes and trust it to stay put ride after ride.
For casual riders or tighter budgets, standard sliding straps work fine—just check them occasionally. But if you're already looking at a higher-end helmet, locking splitters are a quality-of-life upgrade that keeps your fit consistent and secure without the hassle.