It’s ensuring a safe use of essential oils. Diluting them with carrier oils will prevent you from getting severe irritated skin, redness, burns and dryness. Essential oils can also be diluted with water if used with a surfactant that can help emulsify, solubilise or disperse the oils - as water and oils don't mix. Warning: Always remember that where there is water, there is life. A preservative - natural or synthetic - will need to be used on any aqueous formulations in order to prevent the growth of unwanted bacteria and mould. When creating any DIY formulation using water at home it will be better to use immediately or within a short period of time.