Move from time to time; change the pose from time to time. Sitting long in a same pose isn't healthy. During the breaks walk more.
About the usefulness -- this is tricky, you never know what may be useful later in your life. For example, what I learned in the chemistry classes still isn't very useful in my life but what I learned in the English, Philosophy, History, Literature, etc. classes is useful. We really never know what may be of use in future.
Show some examples, what do you learned recently?