So why add this? Well, I try to make the standard keyboard work as well as possible on all screen sizes. However, on phones with a very small screen the buttons may be too small, and on tablets the buttons are larger than ideal. This will allow users to create keyboard to better fit their unique device. In addition, some people just like buttons bigger or smaller than others.
Also, this will allow users to create keyboard for specific purposes. Maybe you are using complex numbers a lot so you want to move the imaginary number i to the swipe down page. With custom keyboards, you can choose to do this. Maybe you are using matrices a lot so you want to add a new row to the keyboard and move those keys to it. With custom keyboards you can choose to do this.
I am very excited about this new feature and can't wait to release it!