There are a few use cases for this, like making a c0.0.14a_08 style world, flat out removing trees, or using a custom tree. This would be useful in some cases where you need something super specific that Modern Beta just doesn't have.
(Also please add a c0.0.13a and c0.0.14a_08 generator if you want to, I would really appreciate it. If not please add some extra advanced settings to allow people to make their own generators)