Offset (back space) is important to maintain handling and steering geometry during turning, almost inconsequential on rear non-steering wheels.
On the steering wheels the offset keeps the tire's tread contact patch and the intersection of the road surface to the "king pin" axis line between the lower ball joint and upper (whether ball joint or strut bearing) close. The further apart the center of the tire contact patch and this "king pin" line are the weirder the handling becomes.
And the "direction" of the distance between center of tire contact patch and the virtual intersection point of road and king pin axis, often called "scrub radius", is critical as well. VW's have traditionally had negative scrub radius (king pin axis road intersection point is outboard of tire contact patch center). The theory was that if a steering linkage failure occurs and the wheels are free to steer themselves, the wheels will tend to toe-in, a safer emergency condition than with positive scrub radius which would result in the wheels tending to toe-out when left to their own devices.
Scrub radius also has some effect on toe relative to braking and accelerative forces. Negative scrub radius will tend to toe wheels in under braking and toe them out under acceleration. Positive scrub radius is just the opposite. And the greater the length of the scrub radius in either direction, the greater the toe effect.
When changing offsets and/or tire diameters its probably wise to consider scrub radius, particularly keeping it negative if possible, even if its reduced from stock spec.