Why are Halbach Arrays useful in PM motors?
A Halbach array motor, no steel:

The shape of the magnetic field in the Halbach array is such that it can close its path if two rotors are used. In a classic motor, you have to use a ferromagnetic material both for rotor and stator to be able to close the magnetic path.
With only one Halbach rotor, you would still need a ferromagnetic stator, with two such rotors, even stator can be made coreless.

As you can see, the flux can have its complete path. In the pictrure there are two Halbach array rotors, in the mid part the stator is inserted.
At first look, it is a miracle of science, since it is lightweight compared to steel rotor and stator. The drawback is that magnets still need a support, could be a aluminium plate, carbon,...the stator also need a support, like epoxy resin. Without the metal parts also the heat dissipation is low, so it tends to overheat.
Yet another coreless motor, full building details