Here's how it works
1) When you home the z-axis, the limit switch will engage twice then the printer will stop on the switch, that marks the HOME, or zero position. Any z-offset you have is ignored here.
2) The printer moves to print the first layer. The head moves up based on the layer height (e.g. 0.2 mm) but there's an additional height because of the gap between the bed and the nozzle when you used the business card for leveling. So the actual nozzle height is 0.4 mm (assuming the card was 0.2 mm thick) above the bed. If you have any z-offset, it will be added, so if you had +0.1 mm z-offset, you'd end up 0.5 mm above the bed. Same with a negative offset, except you will hit the limit switch and/or bed, so that's the practical limit for the negative. Yes, you can plow into the limit switch.
The z-offset is useful because you may find that with different materials and different bed adhesion methods, you'll need to make some fine adjustments. It would be a pain to adjust the bed for that every time or the actual position of the limit switch--cannot reliably move in <1 mm increments anyways. For example, you may find that sometimes the first layer segments aren't connecting. Small negative z-offsets can help that. You can accomplish similar results by using extrusion multipler and first layer width, but I'd argue that's not best practice. It's also handy on laser cutters, where you may want to finely adjust the position of a material with respective to the beam waist.