From a user safety standpoint, when using electrical equipment, it is important to have the accessible portions of the device at a voltage potential that does not endanger the user. When electrical devices are not grounded there is the possibility that a charge may build up, either from environmental conditions, or from the voltages inside. If all electrical devices in use at a particular location are properly grounded there is a reduced possibility of shock. Many governments require grounding on certain types of electrical equipment.
Aside from the safety aspects, improper grounding can affect the operational reliability of a computer system. If a computer system and its peripherals are not properly grounded then excessive ground currents may be generated, which may cause erratic operation. If all the equipment is connected to a common ground then the chances for issues caused by ground loop currents are reduced.