And 3 AEB is shorthand for three shots taken 1 under exposed, one middle exposure, one over exposed. Very high contrast scenes can be problematic for cameras that only have the ability to bracket 3 shots. You can of course do all this manually, but the AEB feature simplifies the process and also makes it so you don’t have to touch the camera while the sequence is taken. (Beneficial for dozens of reasons). 3 AEB is often insufficient for real estate photography or architectural photography if you have to shoot in sunny daylight conditions.

9 AEB allows you to take larger brackets that capture a wider dynamic range. It also allows you to tighten up the EV steps from say 2EV per shot to 1EV per shot and this is beneficial for both reducing noise and taming saturation issues in post.