ACC cover for self employed people

I both work as an employee as well as consult as a self employed contractor secondarily.

How does ACC work for self employed people? I had always assumed my self employed earnings would attract ACC cover. I pay ACC levies on my self employed income as I end up with ACC levies to pay for it. But then I came across this thing talking about self employed people purchasing ACC cover and that got me confused.

Are self employed contractors automatically covered by ACC? Or do they need to buy it like an insurance? If self employed people do need to also buy it liek an insurance then why on earth are we also paying ACC levies as well?