I'm not 100% sure why, but I think the B level is for countries who don't have athletes capable of reaching the A level.
For example, as a country you may be able to send 3 athletes to each race. If you have no athletes capable of reaching the A level, but you have an athlete(s) capable of reaching the B level, then you can send one athlete who qualifies at the B level.
If you have multiple athletes capable of reaching the A level then you can send a full squad of 3 athletes for this event.
I think this is how I remember it being explained to me anyway!