That's known as 'pre -shading', and is designed to subtly bring out panel joints, through the main paint finish, to replicate weathering. If done well, and where needed, from actual references, it can be quite effective. If over done, it makes a model look like a good model, but nothing like the real thing. It works best with acrylic paint for the actual colour scheme, sprayed in thin layers, and is not as effective with enamels.
Personally, I think it's over rated and un-necessary, but it's a matter of personal choice.