IMO, it's mainly there for us. That's why it's in my tanks... And to attach java fern and anubias. It has some other advantages as well. Most, if not all, plecos enjoy munching on it. Some people say that it will lower pH, i've never seen this. I believe that some tannins will lower pH, which may be the case. Tannins also replicate the natural environment of some fish. If you've seen the show 'Ax Men', you'll notice that the Suwannee river has a very yellowish brownish tint to it, with almost no visibility under water. This is because of the tannins released from all of the sunken timber. Some people set up 'blackwater' tanks to replicate this.