With a Scene Unit Scale set to 1.0, the Default Cube is 2m by 2m by 2m. The Clip Start is 0.01m. The Clip End is 1000m.
When the Unit Scale is changed to 0.001, the Default Cube is 2mm by 2mm by 2mm. The Clip Start is 0.01mm. The Clip End is 1000mm.
The reason to change the Unit Scale are for accuracy (there are only a certain number of digits behind the decimal point, that can be represented by a fixed number of Bytes) and it also is easier for us, when modeling very small or very large items.
The Grid is there for convenience. So changing that to the same Scale as the Unit Scale makes it easier to model.
Changing the Unit Scale does not affect Shading, Lighting, Physics or so, but a smaller or larger model does change the behavior of Light, Shading and Physics.
I hope that helps.
