Why does a turn and slip indicator matter?
The turn and slip instruments are combined for convenience and by convention more than anything else, slip is important whether you are in a turn or not. When you slip it means one wing is more directly pointed into the airflow, which increases lift for that wing in comparison to the other. At a high angle of attack, for instance a climb and close to stall speed, a slip can put one wing into a stall, causing the non-stalled wing to flip the airplane. When done deliberately it's an aerobatic maneuver called a snap roll, when it's not deliberate it's a major cause of stall-spin accidents on take-off and approach. So slip is dangerous in some situations. 

In level flight slip creates drag, so it's worth keeping the ball in the middle when safety is not a concern as well. 

As for deliberate slip as I said before slip can be induced for aerobatic maneuvers like snap rolls and spins, slip can also be used for non-aerobatic maneuvers like side-slips, where you put the airplane out of balance to increase drag, allowing more rapid descents without increasing airspeed.