I searched for this in the issue tracker and I didn't find anything. This idea is to allow modders to select which blocks custom plants can grow over. No matter if that block is supposed to have planted anything above.
For example, create a plant that can only grow in deserts. Select that the plant grows over "sand, red sand, podzol"
Another example, create a custom plant that grows in a custom grass block in a custom biome. If you don't want that the custom grass block grow normal plants and tall grass above it, uncheck the "can plants grow on this block?" option, then in the custom biome you can increase the plants' generation value, then add the plant to grow over that custom grass and over that custom biome and voila! You have the biome that only grows that custom plant in it. (Sorry if I extended this a lot)
A more simple example, you can add a plant that grows over bricks and stone bricks.
This also affects where the plant is supposed to be placed. In the last example, if I rightclick on grass, it will not be placed, but if I rightclick on stone bricks it will be placed.
Thank you for your attention :D