Cordless mowers have many advantages over their gas powered counterparts. For example, there none of the hassle and mess of having to deal with gasoline and oil cans. Starting is considerably easier as there is no pull cord to contend with; from experience some gas powered mowers can be very temperamental at starting, particularly towards the beginning of the season. Another factor is noise in that cordless mowers are significantly quieter so no need to worry about disturbing the neighbors.
While corded electric lawn mowers possess many of the advantages referred to above, they are also rather limited by the fact that you have to be plugged into your power source to use it. Therefore you are limited as to how far you can go from your house. Whilst you could use extension cords, this clearly has safety implications.
Cordless lawn mowers are therefore a great solution for anyone who is fed up with the hassle of gas powered mowers, but do not want to be limited by having an electric cable trailing behind them as they are mowing the grass.