By concentrating the magnetic flux between two opposite magnet poles, and capturing the flux in iron plates that would otherwise be wasted, we direct as much energy as we can through the gap between the faces.
The set of rotors (above) uses round magnets.
This is common on smaller axial-flux alternators, but as they get larger, it is often more practical to use rectangular magnets, which are available in larger sizes, and the wire coils are more compact. It is important that the rotors be made of steel or iron, so that the magnetic flux is conducted by them.
The magnets are arranged in a N-S-N-S pattern around the circumference of the rotors. Opposite poles face each other. If you trace the lines of flux, they travel from one magnet face, straight to the magnet face opposite, then travel through the steel rotor plate to the next magnet, and back across the gap. Coils of wire in the gap capture the magnetic energy in those field lines.
Flux lines traced through a dual-rotor permanent-magnet alternator
The path of magnetic flux should be more clear with the diagram above. The flux has been concentrated by confining it between the plates. The flux also alternates between North and South. A compass inside this gap as the rotors turn would flip back and forth frantically. A compass outside the plates is weakly affected, because the fields have been confined.