The tea is mostly a nutriënt source. Like you said, it stems from plants, so the leaves are a rich food source. It contains carbon (sugar) and nitrogen (amino acids), as well as some trace elements (minerals, not necessarily needed to survive, but that have a big effect on for example how efficient enzymes work).
Furthermore, it contains caffeine, which could be working positively as well (some micro-organisms don't like caffeine, or even die, but others endure no effect). There is an interesting paper demonstrating that caffeine might even lengthen the lifespan of Saccharomyces cerevisiae, which is baker's yeast and the one you'll find in kombucha. This is the link:
Some people make kombucha with coffee, but it does not produce a new scoby! And, you can't reuse the scoby to start a new batch:
Kombucha is actually a pretty complex ecosystem, so who knows exactly which type of organisms use what parts of the tea...