People who use computers all day can have many problems. Hand, back and neck can all ache. The eyes can smart, become sore and water.
It is known that all people, after about 30 minutes, start to lose concentration. It only takes a very short time to break the concentration and come back refreshed. It could mean just gazing out of a window for a few minutes, watching the clouds. Watching the clouds has other benefits. The eyes, which become adjusted to the monitor and therefore fixed, can refocus on something distant, giving them much needed exercise.
We collapse the more we stay seated, so in taking time every 20 minutes or so to stand up and sit down again, means less collapse and therefore less strain on our necks and backs, our arms and hands. Of course it is important that the keyboard is in a good position, the monitor is at the right height and our chair is at a height that enables us to put both feet firmly on the floor.