Hehe…good question and sorry for being so vague about it^^ It works for all meshes…not just trees or vegetation. But its more efficient the more complex the asset is to lightmap.
What it does is this: right now…probes are only being generated around static geo. But they are not lighting static geo cause that uses lightmaps. The probes light dynamic objects that can be close to that geo and should receive bounce from it.
If you set a static mesh to dynamic, it will receive lighting from the probes, but the probes will not hold any information about that objects itself, since dynamic objects are not baked into probes.
So what this mode will allow you to do is to set a static object to receive the lighting from probes that will contain information about that object while not using any lightmaps. Thats like the most efficient way for now to light complex objects.
Many games use probe only solutions cause the datasets are better and you dont need lightmaps.
To name a few, GTA, Horizon Zero Dawn, all Assassins games, Farcry etc. They are all fully probe lit. Since probes interpolate between them though, you can get light leaking inside interiors, which is why a combination of lightmaps for buildings and probes for vegetation etc. is one of the most efficient solutions while still being visually pleasing 
Hope that clears things up