When I add lightmap in my material and then remove it, material seems to not be affected by shading. If I overwrite this material , only way to fix it is create a new material from scratch.

Check that when you add lightmap, the "rgb" parameters (ambient light) go to maximum, so after removing the lightmap you can set them to zero.

Ah, I'm really blind Thanks for reply. Could you explain me in which situations lightmaps is a good solution? What exactly lightmaps do?