In my fantasy world, in a time when civilizations still didn't exist, there were some formless creatures with the ability to change in every way they wanted. Many of those creatures with the passing of time stabilized in very different forms depending on the kind of evolutionary path they decided to take. Some of them bonded with magic, others with the ruins of an ancient technology, others decided to imitate the beasts of that world, and so on. And I'd like for some of them to become tree-like people too. But what would be the advantages of taking such an evolutionary path?
I thought that maybe they lived in a place where nourishment could be absorbed only with root-like appendages from the soil, or that becoming vegetable-like allowed them to establish a symbiotic bond with fungi or other useful lifeforms (but which ones could justify that choice?). Nevertheless, even just having a bark-like skin seems quite inconvenient, so why should they choose to evolve like that? I like the idea but I don't want for it to sound too forced. Do you have any suggestion to justify such evolution or is it better if I discard it?