Why do planes need a roll motion?
This answer is specifically for a fixed-wing airplane, not a helicopter or multicopter, although some aspects of it will apply to them too. 

What is a turn?  A turn is a curvature in the flight path.  This means a turn is a form of acceleration.  Specifically, a centripetal acceleration-- the flight path is being continually "bent" toward the center of the turn, creating a circular path.  

Newton's second law is force = mass * acceleration, or acceleration = force / mass.  You can't have an acceleration unless you are exerting a force to cause that acceleration.  You can't have a centripetal acceleration unless you are exerting a centripetal force-- a force vector pointing toward the center of the turn.

What part of an aircraft is designed to produce force as efficiently as possible?  The wing.  If you roll the aircraft into a bank, you tilt the wing's lift force to one side, and you've now a created a centripetal force as efficiently as possible-- i.e. with the least drag possible.

Instead of banking, you can also turn by yawing the fuselage sideways relative to the flight path to expose the side of the fuselage to the airflow.  Essentially this makes the fuselage act as a sideways wing, generating some sideways lift or "sideforce".  This method also points the thrust vector from the engine or engines sideways relative to the flight path.  So this method will create some centripetal force, but it is very inefficient-- drag will be very high.  It will also be uncomfortable for anyone in the plane as they will tend to be thrown against the "upwind" side of the fuselage.    

Note the difference between causing or allowing just enough yaw rotation to let the nose track around the horizon and "keep up with" the changing the direction of the flight path as the flight path curves due to the centripetal force generated by the banked wing, and forcing the nose to yaw to point in a different direction than the aircraft is actually travelling through the air at any given instant, as would be required if you were trying to turn without banking at all. 

A car on flat ground can turn without banking fairly efficiently but that's only because the tires usually grip the road quite well, so it only takes a very small sideways angle between the alignment of each wheel, and the instantaneous path of travel of that part of the car, to generate a strong sideways force.  So you don't scrub off a lot of energy due to excess drag.  Very different from dragging a fuselage sideways through the air at a large sideslip angle.

A good source for starting to learn about the basic physics of flight is John S. Denker's "See How It Flies" website .  On second thought, this website is not so much designed to introduce a complete novice to the basic principles of flight, as to help someone with some flying experience and training understand the "why" of it all on deeper lever than some pilots ever manage to reach in a whole lifetime of flying!