Soil-less growing is the alternative to growing in soil or hydroponics. Some soil-less grow systems could be considered hydroponic. An example would be a top feed system in rockwool cubes placed on rockwool slabs. I would also consider a container plant using hydro clay as the medium would be a soil-less grow. These systems can be easily maintained using a water pump to feed water and nutrients to each container or plant.
There are a multitude of reasons to choose a soil-less grow over a soil grow and vice versa. First, soil can be VERY messy. Accidentally knocking over a container that is pre-filled with dirt for a fresh transplant costs minutes of cleaning and a loss of soil. You can also contaminate your soil with whats on the floor if you try to put it back in the container. Soil is also a nesting haven for a lot of different bugs like gnats and crawling insects. Gnat nests buried in soil can be hazardous to your plants. The bugs can eat at the roots of your plants without you even knowing.
Simply switching the growing substrate to hydroclay or hydrorocks can help to control the problem. There are a wide variety of mediums to choose from when growing soil-less. There are also rockwool grow cubes, coco coir, and perilite. You can always mix a variety of things to create your own soil-less mix to create the environment you want for your plants root system.