Why is soil drainage important?  Because most plants are not suited to constantly having their feet (roots) wet. Typically, plants need to be watered and then let the soil dry out before being watered again. Having their roots too wet causes the roots to be vulnerable to certain root diseases that will kill the plants. So having soil that drains well is important. If your dirt is hard packed, it probably won't drain well and you should mix in peat moss, compost or potting soil to promote drainage.