Binders are needed to keep a pill shaped like a pill. You can't just press any old drug into pill form - it may not have the physical properties to hold its shape, it'll just crumble in your hands. Often, if the doses are small too, it needs to be bulked out so the pill is a standardized side.
Sometimes the binders/fillers help absorbtion, speed up disintegration of pills, increase solubility, add flavor, deter abuse/extraction, preserve against spoilage, or provide extended/delayed release.