The purpose I think is to trick our eyes.

With ambient light, our pupil will constrict a bit, as compared with full dilatation in total darkness. In full dilatation, it absorb more light, as a result, the dark LCD will be more 'greyish', so leads to lesser 'black' in a dark scene.

In addition to that, usually bright LCD image in a dilated pupils will give rise to more visual fatique for the viewer.

So ambient light at the back of LCD will not only constrict our pupil, providing more black imagery in dark scene, also it gives less visual fatique for prolonged viewing.