Why is it important for a student at a university to be engaged with his or her studies?

The reason that it is important for students to be engaged
with their studies is that they are much more likely to succeed if they are engaged. 
Since college performance is important to one's future prospects, engagement in one's
studies is extremely important.

When people get to college,
they are typically facing an academic environment that is much more rigorous than any
they have yet faced.  At the same time, they are faced with a social situation with more
freedom and temptations than they have had while living at home.  Since the work is more
difficult and since it is more difficult to concentrate on that work, it is vital that
students be engaged so that they can have a better chance of doing well in their
classes.