Perhaps you didn’t know this, but plants are usually more than 80% water! They use this water similarly to how we use blood in our bodies – it’s there to transport nutrients from the soil to the plants. If you cut off that water supply, you essentially cut off all nutrient intake from the plant.
To sum up, a self-watering planter is a device that ensures a constant flow of water for the plant’s roots. This way, your plant is always properly hydrated and you don’t have to fear that it will run out of water at any point.