ET or Einpressteife, or Press depth, offset or rim offset just affects the way the wheel sits in the arch. Either closer to the arch or the strut.
Technically, lowering has nothing to do with it, until you fit wheels that are too wide/too low an offset, that can foul the arch when the car is lowered.
ET can satrt to affect your handling however as on the steered wheels, the offset affects the roll radius/scrub angle.
The further you move the wheels projected centre line away from the hub face, the more leverage you will introduce and suffer bump steer