Why are rain trees often found in gardens?

Because there aren't enough forests in the world to grow them in.I'm not sure what your asking. If your refering to a Laburnum anagyroides or Golden Rain Tree then the answer is probably the beauty of the tree when it is in bloom. The tree itself is rather small at 25' and in late spring, early summer it gets covered in 6-10" long yellow flowers,which looks like the same type of flower as wisteria. Just remember, please, all parts of the tree are VERY poisonous and should be handled with gloves