These trees are for open scenes or whatever you want to do with them. It comes with only four models, but there are three textures for each tree. And with each model having the same UV mapping, you have 4 models times 3 textures for 12 trees! Have you even seen 12 models in a store for such a cheap price? I sure have not!
What would a forest be without trees? It would not be a forest at all, but a meadow. Whether you are creating a forest, a city, a medieval village, or even a sci-fi scene you need trees. And this will save you time, especially if you are developing a game and have a tight deadline. And you could use the models for future projects as well.