Grounds on circuit diagrams can be there for practical reasons or for convenience, or both.
Note that not all circuit grounds are actual Earth connections -- that is, not all grounds have a physical connection to the Earth. They may connect to a metal chassis (or circuit container) or just represent one of the common power supply rails (usually the 0V supply reference point, particularly for circuits with multiple supply voltages such as +/- 15V, or 12V and 5V, and so on).
When used for convenience they represent a common reference point from which circuit potentials can be measured, and are treated as though wires were drawn connecting them all together. This serves to unclutter diagrams, especially where if the wires were actually drawn in they would cross over other wires and make the drawing harder to understand.
The practical reasons include shielding (from stray electromagnetic noise), polarity recognition for interconnecting circuits, and safety features. Actual Earth connections are important for safety reasons so that equipment potentials don't 'float' far from Earth potential and create shock hazards when people come in contact with the chassis or controls; This is true particularly when AC mains voltages are involved. Actual Earth connections can also be important for high performance antenna systems, where a 'pool' of charges at constant potential (Earth ground potential) is helpful for allowing maximum current flow in the antenna circuit in response to induced signal voltages.
Circuit grounding is actually a pretty complex subject, and in some cases is more of an art than a science particularly in sensitive circuits where noise suppression and avoidance of exotic feedback paths is necessary. Google "ground loop" or "phantom loop" for a taste of the topic.
