BSA is the traditional threaded bottom bracket standard—think of it like screwing a jar lid on and off. The cups thread right into the frame, which makes installation and removal straightforward with basic tools. No special press required, no trips to the bike shop for a swap.
For gravel riding, BSA has a real edge. You're dealing with mud, stream crossings, and long days far from home. Threaded bottom brackets resist creaking and are far easier to service yourself—just unthread, clean, grease, and rethread. That matters a lot when you're bikepacking or riding through sloppy conditions.
The trade-off is a slight weight penalty compared to press-fit options, and the narrower shell means a bit less frame stiffness. But for most gravel riders, the reliability and easy maintenance more than make up for a few extra grams. You'll find BSA most often on steel and titanium frames, where it fits the no-fuss, built-to-last philosophy.