The purpose or function of an object limits its possible compositions and structures. If a hammer is needed but there are none to be found, there are some other things that could substitute but only if they are made of the right material and have the right shape. A pillow is clearly not hard enough and is too big and mushy. A can of soup might be hard enough but it would be really awkward to use because it's so wide. A coffee mug would be hard enough but might break. Really, the reason hardware stores can sell hammers is because there is nothing quite like them that does the job so well.
The same principle applies in biology. Every structure that exists, both at a macro and a micro level, exists to fulfill a certain function and that function places limits on the structure. We can use the same principle backwards. If we don't know the function of a structure, we can often infer or guess what it might be, based on the structure's shape and material makeup.
An anteater's long "nose" is not actually a nose but a mouth. It is totally reasonable to assume that the unusual shape somehow helps the animal eat. Anteaters claw open anthills and then stick their long, skinny mouths down the hole while they lick up lots of ants with their long, sticky tongues. The structure fits the function.
This phenomenon is frequently used by taxonomists when comparing organisms and classifying them. It is particularly helpful in learning about extinct organisms that we can no longer observe in action. Their fossils and skeletons can give us a lot of information about their manner of locomotion, for example, which then might tell us something about their habitat, too.
Look at the fossil below. What is it?
A fossil of Ailuravus macrurus skull, an extinct animal. Image from here.
It's extinct, but what might it be related to? First clue: its tail. It's long compared to the size of its body. What other animals do you know like that? Mice, rats and the like, right? What taxonomic category are they in? They're in the Order Rodentia, the rodents. Could this be a rodent? What is another characteristic trait of rodents? (Check out this clip about the filming of R.O.U.S.s [Rodents Of Unusual Size] in The Princess Bride.) Big front teeth for gnawing on things—every rodent has them. Does this guy have them? Hard to tell from this picture but it definitely has an overbite, which suggests big front teeth. It also has short front legs and longer back legs, which also fit the rodent way of scampering. In fact, all of these guesses go right along with what scientists think. Scientists have decided that this was probably a relative of modern-day squirrels. They named it Ailuravus macrurus. This was all figured out by looking at the bone structures and guessing their functions. No need to see the furry body. Fossils give us prehistoric X-ray vision.