Certified GKH Gardening Expert
Think of it, sort of, like a potato!
They do grow tubers when they are happy, so that they can preserve extra nutrients for when they become unhappy. There is conflicting evidence of trying to propagate these, and I've come to the conclusion that they are solely for nutrient storage, not propagation.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/houseplants/prayer-plant/maranta-prayer-plant-peacock-plant.htm