A satellite constellation is a network of satellites that provide global or near-global coverage. For the communications field, the satellite constellation allows satellite services to continue uninterrupted anywhere on Earth since at least one satellite will always be visible to any receiver. Constellations are ground-supported by a network of ground stations that are globally connected via fiber or other terrestrial services.
The reason LEO is used over GEO for constellations is the difference in coverage area. GEO satellites, being over 22,000 miles above the Earth’s surface, cover a large area of Earth when transmitting data and therefore gain little from a constellation. LEO satellites, being around 1,200 miles above Earth’s surface, cover a small area of Earth when transmitting data. LEO satellites compensate for the small transmittance area through constellation and faster uplink/downlink. It could be said that a constellation is required for a LEO network to operate successfully on a global scale; this is not the case with GEO. LEO satellites also have shorter transmission delay than GEO around only a few milliseconds. GEO may have 100 millisecond delays or longer due to the longer distance the signal must travel.
LEO satellites have a very short orbital period: that is, a LEO satellite takes only 84 to 127 minutes to orbit the Earth once. Since LEO satellites travel quickly and are located close to Earth, limiting aperture field of view, the time window available to achieve uplink/downlink is small. Having a constellation improves both effective field of view and the window available for transmission.