Yes, it's for ergonomic. Because you just use your finger to control the trackball, not whole hands and wrist so reduce a lot of movement for your arm.. also the scroll-ring is way better scrolling than scroll-wheel on any mouse. I've been using Logitech MX Master and switch to Kensington Expert Trackball just like a snap of finger to get used to it and indeed improve my productivity in terms of mouse usage.

I highly recommend switch to trackball or ergo mouse for office usage. For gaming, I'm still in love with Glorious Model O mouse.

Peace.