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.
- More keys for the thumbs allows for using the thumbs more instead of the (weaker) pinky fingers.
- I like symmetrical keyboards; it's more comfortable than the standard row-stagger.
- The custom firmware has some nice features which allow for reducing hand movement. e.g. putting a numpad layer where your hands rest. e.g. I have "go to prev/next virtual desktop" when I hit the keys "cv" or "m,".
- The keyboard firmware is on the keyboard itself. This is nice if you switch between different computers.
- More desk space for e.g. notes between the split halves.
Disadvantages:
- In the same way that e.g. a Vim config can be ultra-personalised to the point no one else can use it, the same applies to split keyboards.
- Mechanical keys can be quite loud.
- The keyboards are either DIY, expensive, or both. They can be quite difficult to acquire.
- It can take some time to adjust to a new layout. Moreso if you've not done per-finger typing practice with e.g. Dvorak layout.
- It's more difficult to use with "keyboard + mouse" programs like Blender, CAD, or videogames, which use QWERTY keybindings.
- As with any external keyboard, difficult/awkward to use with laptops e.g. at a cafe.
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.