I came across this idea on Pinterest and, I must say, as a lazy gardener, this could be an excellent way to turn parts of your lawn into garden with very little effort.
Place the bags on the area of lawn you want to kill, slice them open and plant away. I would probably also make some holes in the bottom side of the bag for drainage. By the end of the season the grass should be dead and ready for removal and you already have a handy amount of topsoil to fill the whole left by the sod.
I could see using a similar method to start converting some bare areas and unneeded garden paths in my garden. I don’t have any grass here, so it would be more a method of expanding overall gardening space than carving the space out of a lawn.