Growth media is used throughout a plants life cycle and can be made of anything that simply holds a plants root structure. This includes Rockwool, vermiculite, coco, perlite, clay pebbles and soil mixes.
Different growth medias have different benefits. Some come with essential nutrients and minerals already infused, others are completely inert – giving you more control over what nutrients are given to your plants. Moreover, some differ in their air capacity or ability to keep hold of water.
Different hydroponic systems need different growth medias to work at their optimal ability - what you use depends on what your system needs.