as you see, this image is pretty sharp, it has good lighting, and some ambient effects, but no texture and you still
see what it is supposed to be.
Well, not so much. You can see the shape (that's where lighting helps the most) and you realise it's some kind of terrain (old-school, minecraft-like), but you don't know whether it's desert, grass, dirt, rocks. Or even Earth or Mars?
You can be perfectly fine without textures, but you'll get a specific look (which can be great for games, these days it's kinda popular to come with new or simple looks), but most probably it won't be realistic. Stylish, not realistic.
Textures don't provide only color information, but also additional small details which you would have to model via polygons otherwise.
And also colors aren't solid in real life. You can make brownish block, but people won't realise it's supposed to be wooden plank without a proper wood texture.