Why is acoustic plasterboard taper edged only?

I was thinking of using acoustic plasterboard to board a wall that has a coom ceiling and Victorian dormer window, and noticed it’s only available with a taper edge.

Why is this?

How do you deal with outside corners when using taper edged board? Does two tapered boards meeting at an outside corner not cause problems for the plasterer? (It will be eventually all plastered).

Thanks,
Ian

The joints are filled first before skimming ... otherwise noise would pass through the joint