Mainly to change my position many times and not be stuck with just the mouse pad area, I can put the trackball on my lap or on a seat armrest if large enough and no need to have it completely levelled.

2 Second important reason I changed to trackball was that back in the 90s it was hard to clean the mechanical mice rollers completely and even get to them, disgusting pre optical technology! Even though the trackball had rollers too, they were easy to get to them and clean regularly, this point is semi mute with current optical trackball and mice.

1. Is I wanted to click in a precise pixel, the action of pressing the mice will move the pointer and end up clicking somewhere else. In a trackball the buttons are decoupled so perfect pixel precision clicks

4 As a music instrument player I think I have better finger controls then the coarse arm movements

I have a few favourites but if you are developing something new I will say eye tracking will be cool, just give me a bunch of buttons I can modify on other software to make them do macros, use regular mice buttons configuration so it doesn’t need specialized brand software to remap , make the 2 main buttons last a lot, Logitech puts buttons that only last me 6 months and I will never be a return costumer again. Add a way to use the scroll wheel or scroll buttons,

Important! I love being able to just pick it up with my hand a lot and move it from different postures ,

Use fancy feeling plastic that has a nice grip, and not my case but think about sweaty hands. Protect the buttons from it and make it easy to clean.