Clones and seedlings don't need much other than water and sun because all the plant is trying to do is produce more roots. To do that at such a young age, all your plant needs are the basics. Now, it should be noted that in nature seedlings also have the benefit of being surrounded by soil, so some seedlings will receive a bit of added nutrition. So too will growers use a tiny bit of nutrition in their rooting clones for a bit of added growth at a young age. But as long as the environment around the plant is sufficient and it's receiving the right amount of water and light, it'll be able to start growing. Giving your plants nutrition before they're ready can shock them and cause them to become sick. They're not able to process the nutrients they're receiving, and the elements that should be giving them what they need to grow will start to harm them. Other than in their early stages, though, plants will need more nutrition than water and light. Think about any living thing: it can survive and grow with the basics until it reaches a certain age, and then it will need more than things like milk or water. It's the same thing with plants: if they don't get nutrients while they're growing they'll either grow into weak plants or will die off early. But that doesn't necessarily mean you have to start adding nutrients to your feeding schedule- not yet anyways... Why and When Plants Need More Nutrition As any organism gets older it will require more complex nutrition. Think about us humans: we can survive on milk only so long before we start needing to consume other vitamins. Plants are no different. After a couple of weeks, plants will require more than just water and light to grow. They, too, need more complex elements in their diet to grow, and moreover, to breed and grow fruit (which is a form of propagation, but we'll get into that later). Now, that doesn't mean you have to run out and grab bottles of nutrients. If you're careful, you can continue to use nothing but water in your grow for a little while longer- if you're growing in the right medium.
To have the best shot of giving your plants nothing but water you'll want to be growing in soil or aquaponics. These two mediums can release nutrients to your plants naturally instead of depending on someone adding those nutritional elements into their water supply. It should be noted that not any ol' dirt will work for add-free plant growth. If you're attempting to use nothing but water to grow your plants, you've better have some supercharged soil ready for your plants. Only when your plants have all the nutrition they need in the soil they're in can you feed your plants simple water. However, you'll want to make sure to level out the pH of the water used and be prepared to use a reverse osmosis treatment on water before feeding it to your plants. In an aquaponic system, fish are quite literally fertilizing your plants at all times of the day. From the food they eat to the waste they produce- even down to the scales they shed- an aquaponic setup is the only additive-free way to grow because anything the plant needs is being produced by what's swimming around under plant roots. Bottom Line: Plants Need Nutrition So can plants survive on water alone? No. All living things need nutrition to live and thrive. Whether your plants get its nutrition from the soil, from animals, or from synthetic additives, to produce a plant at peak condition you'll need much more than water and light to get there.
Different elements will help grow various parts of the plant and add to the structure and growth of the plant overall. From root growth to nutrient absorption, leaf production to terpene creation, plants need a complex set of elements to grow to its best state. That's why if a plant's only receiving certain types of elements but receiving little to none of the others, you'll see breakage of leaves, weak branches, why nutrients won't absorb as they should, and a whole host of issues that come with the lack of nutrition. If your plants produce seeds it will most definitely need nutrition to get to harvest. Giving them nothing but water- or by using old, nutrient-barren soil- it will grow scrawny and give you little to no yield. So if you want to use nothing but water in your grow, you'll have to make sure your plants are getting nutrition somehow. Eventually, water alone just won't cut it.