Let me put it another way.
Wheel offset is the more important of the two measurements. It is sometimes possible to install a wheel on a vehicle that has enough backspace such that it doesn't interfere with the steering or suspension components yet has dangerously incorrect offset.
In such a case, the weight of the vehicle will not be acting on the wheel bearings at a vector specified by the car maker. Think of the wheel (which is supporting the car's weight) as a lever acting on the bearings and spindle. Too far in or out from the car maker's specs and the bearings may fail, spindles may break, etc.
