The benefit of leca is to give the water somewhere to drain to. It essentially becomes a reservoir similar to a false bottom. It will outlast your viv. Without it, your soil would be the bottom layer, and as water through misting and watering plants would saturate your soil, creating a bog and that comes with mold and bacteria you do not want in a viv.

Keep in mind, you will need to leave yourself a way to drain the leca from time to time.