Answer: It's a long explanation, but basically, roots need both water and oxygen. Plants that will root in water can actually use the oxygen that is in the water and the roots will thrive if you change the water every few days. As you put fresh water in, you are also adding some fresh oxygen. Some plant roots get along with less oxygen than others. Examples of these types of plants are those that you might find in the saturated soils around a lake or a pond. Not all plants are suitable for growing in soggy soils or rocks surrounded by water, but many are.
Many houseplants can be rooted in water (pothos, wandering jew, even an avocado seed) but eventually a lot of roots grow and a small container will not hold enough water to provide oxygen to all the roots, so the plant will start to decline if it is not potted up.
Hope this explains things!
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