why is it useful to know the net force?

Answer 1

The net force will tell you whether or not the object accelerates, how much acceleration it will have, and which way it will accelerate.

If the net force is zero, the object is not accelerating. Note that it could be moving at a constant velocity though.

If you divide the net force by the mass of the object, you will know the amount of acceleration the object is experiencing.

If the net force is non-zero, than whichever way the net force is directed, that's the way the object is accelerating.

Note that objects do not necessarily accelerate the way they are moving. For instance, a ball thrown upward has a net force (gravity) that is directed downward. So even though the ball is moving upward, its acceleration is downward. So the net force is the best way to answer questions about which way an object is accelerating.

Answer 2

Knowing net force is important if you want to calculate how much push or pull it takes to move an object. Like in cars or planes.