Without the BB course you could go straight from the RPAL course to the IPSC course without ever having fired a gun. This would likely result in you being 2 things.
1. Unsafe. No idea of expected gun handling, safety areas, range commands, etc. and you would either be a guaranteed DQ or worse.
2. Hopeless. While we absolutely encourage new shooters and would never mock or disparage someone for poor performance you wouldn't have a hope of being competitive, and while there is no shame in coming last there is some discomfort in doing very badly, it can be excrutiating to watch a shooter really struggle, and it eats up valuable time when you have lots of shooters, lots of stages, and one shooter takes forever to complete.
Its not reasonable for a club exec or RO to just let any guy with a gun blast away on their range. Part of the exec's job is to protect the club from accidents and litigation, and that means some system of vetting unknown shooters.
Its also not fair to take someone's money and allow them to participate knowing they are going to tank, DQ or otherwise embarrass themselves, so training and prequals is good.
Can you think of any game or organized sport where you can play without effecting others having zero skill or not knowing the rules..its expected they would be taught first. Its tough to play soccer with a guy who thinks its like rugby and keeps picking up the ball.
I also think, personally, I want to shoot IPSC with people that have made some sort of commitment to the sport,and that commitment is the BB course. There are plenty of fun matches that don't require IPSC level of training and commitment, and I feel IPSC was designed to separate the casual from the more competitive shooter.