The chair will help keep the rest of the back and neck erect and decrease your chance of hunching forward, which can cause spasms in the back and neck and headaches.
The main problem appears to be bad posture when using the laptop on your, uh, lap -- hunching over in this way apparently causes chronic back problems in all sorts of people.
The chair will help keep the rest of your back and neck erect in order to decrease your chance of hunching forward, which can cause spasms in the back and neck and lead to headaches.