Plantpeople are just podpeople without the "healing in light" mechanic. I believe they were originally included in case someone wanted to develop them into a roundstart race, but since no one's done that they've just been doing two things in the code:
1. Tricking wizards who use the magic mirror into picking plant over pod and then getting confused why they aren't healing.
2. Making turning into a plant based human slightly too likely in the staff of change.