[Ndymium]

The biggest advantage is freely positioning the halves to suit your personal preference. You might have them in a /\ arrangement next to each other, or | | so that they match the position of your shoulders. I'm currently sitting in a rocking chair with my laptop on a small desk and the halves of my split keyboard on my legs, which works surprisingly well. When I'm on the move without an ergonomic desk, I tend to put my laptop on a cardboard box to avoid having to look down all day, and my keyboard halves on each side. It helps me avoid neck strain.

For me, getting a split keyboard was a matter of protecting my hands. I felt strain in my wrists and fingers when using a regular keyboard all day. With a split keyboard my wrists can stay straight and I was forced to learn the correct finger positions. I no longer use a mouse, but instead my keyboard's mouse layer keys, so I have no reason to move my hands away from the home position.

Of course most of the split keyboards, being made for enthusiasts, also contain a lot of other features like customisable firmwares and such.