Assisted GPS (A-GPS) is used to speed up start-up times of GPS-based positioning systems. GPS may have problems getting a lock when the signal is weak and in such a case A-GPS would assist in getting a lock.
Standalone/self-ruling GPS devices depend solely on information from satellites. A-GPS augments that by using cell tower data to enhance quality and precision when in poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit multipath propagation where signals skip off structures, or are weakened by meteorological conditions or tree canopy. Some standalone GPS navigators used in poor conditions can't fix a position because of satellite signal fracture and must wait for better satellite reception. A regular GPS unit may need as long as 12.5 minutes (the time needed to download the GPS almanac and ephemerides) to resolve the problem and be able to provide a correct location.
An assisted GPS system can address these problems by using external data. Utilizing this system can come at a cost to the user. For billing purposes, network providers often count this as a data access, which can cost money, depending on the tariff.
Some sports watches with GPS only and without A-GPS will lack accuracy when operating in areas with many tall buildings, canopy. While most smartphones today have A-GPS. That's why the GPS on your phone often has more accurate results than on sports watches.