Normal maps represent a vector. That vector should have a length of 1 in order to be applied properly onto the mesh.
When doing several manipulations to the normal map (adding them together, blending them, etc) it is quite possible that your image no longer represents a valid normal map.
This node will unpack the normal map into the full -1 to 1 range, normalize that vector and then move the results back to a 0-1 range.
Note
Replaced by Normalize Vector COP recipe in Copernicus. To be removed in Houdini 24.0.