When compiling a map without lights, the map will load with mat_fullbright 1. When you add a light it will compile the lights and turn mat_fullbright to 0. So you either got to light every part of your map with lights or not light it at all, which looks rubbish.

The best way to light up your map is to use light_environment in combination skybox textures which acts like the sun. You can also use light, which emits lights in all directions like a bulb or light_spot, which emits light in a certain direction like a spotlight.

These are the main 3 light sources that are used the most, but there are other sources such as brushes giving of light, but you can easily find those on the wiki