When I'm trying to identify a willow observation, I may need to see the underside of the leaf, but too often no such view is provided. I can't reach through the internet to flip that leaf! I depend on you, the observer. Please flip it for me. Please. Below are some examples where seeing both sides is important.
Leaves. Show the upper and lower surface. For some species, we'd like the view of the lower surface to be close enough view to see the hairs. Of course, the camera may not cooperate with that.
Flowers. We all love beautiful photos of the front of a flower. However, identification may require information about the sepals or phillaries or the flower shape, so also get a photo of the side or back of the flower. (And don't forget to include leaf photos! The flower may not be enough for ID.)
Galls. Identification is much improved if we can see how the gall is shaped on both upper and lower side of the leaf. So flip the leaf and photo both sides.
Butterflies. Both the upper and lower side of butterfly wings may be useful for identification and if you don't know what genus it's in, you don't know which side is more important. So photo both sides if possible. Obviously, you can't flip a live butterfly over, but if you wait a bit, the butterfly may move enough that you can get a photo of top and bottom. (Sometimes the butterfly just will not expose both sides. Go ahead and post what you have. This is often enough for ID, but not always.)
Dead birds, snakes, turtles, frogs, etc. Animals are rarely polite enough to die with their most identifiable side up, so it the corpse is relatively fresh, photo the obvious side and then, please, use your foot or a stick to flip it, then photo that other side. Please do NOT flip and post really gross dead animals. (Or if you do post them, post something non-gross as the first photo, even if that's just a cropped picture of a bit its tail or toe.)
Mushrooms and shelf fungi. Seeing the top surface is important, of course, but we need to know if the fungus has gills or pores, and sometimes details of those structures, so please show the lower surface. A photo of the stem of the mushroom may be useful, too. You may need other, microscopic features, too, but that's another issue.
Lichens. Identification is likely to require seeing both top and bottom of any flat lichen.
Snails (including sea shells). Take three photos, showing the top, the bottom, and the side. The side photo should show the opening where the snail body is or was.
Slugs. Besides the top view, get one of the right side, the side with the opening slugs that breathe through. In a few cases, you need a photo of the bottom of the slug's foot, too, though I admit I usually don't disturb the animal that much.
Clams. If the clam is alive, make sure you get the side and the hinge area. If you're dealing with empty shells, get the outside and inside, making sure you show well the area near the hinge inside.
Different species may have different requirements for identification. In general, get at least two photos when possible, including a general view as well as close-up(s). That said, if you have an idea what it is, you may know that one view is important and others are not. We'll always try to ID with what you give us, but sometimes we do need your help to see more of the organism.