Well, you don't have to use PWM... but if you do, then you will have control over the Brightness and will be able to adjust for your own comfort level. If you have two PWM channels available, then you can do the same thing for your LCD's Contrast. Search the forum for Easy Contrast if you want to do that.

If you just want to turn your LCD's Backlight ON/OFF then any I/O Channel will do, but you must know how much current the Backlight will require... if it requires more than 25mA, then you need an external Driver of some sort.