From the documentation :
What Types of Meshes Should Nanite Be Used For?
Nanite should generally be enabled wherever possible. Any Static Mesh that has it enabled will typically render faster, and take up less memory and disk space.
More specifically, a mesh is an especially good candidate for Nanite if it:
Contains many triangles, or has triangles that will be very small on screen
Has many instances in the scene
Acts as a major occluder of other Nanite geometry
Casts shadows using Virtual Shadow Maps
So the documentation is very clear to not use it if using only low poly geometry and without many instances (it’s good for a Lego/Minecraft-like game too with thousands of same instances)