When we talk about non-geo-stationary satellite systems, the advantages are that lower transmitter power are required, because now you have to cover only a smaller distance and, since there will be more that one satellite in any particular orbit, you could use higher inclination of the satellite for connectivity.
They also have got higher operating reliability, because of the additional redundancy that we have, and because these satellites are typically also crossing the South and North Poles or the regions near that, the higher altitude regions can also be covered, and since they are at a smaller distance from the surface of the earth, the propagation times are small.