Close your eyes. Try to imagine what the inside of your hand is like - the bones, muscles, ligaments, tendons. How do they look? What are they made of? Are they made of different things? How do they all work together?

It's pretty difficult to imagine all of that in your head, without being able to see something to relate it to - a model of a skeleton, or even just a drawing on paper. Being able to study something you can't actually tangibly measure or see is one the hardest things about studying science (nearly any science). So, we use models and diagrams to ascribe some order to things and to create a tangible construct that we can then use to communicate our thoughts and ideas.

Here's a more in-touch example: Say you play The Sims 4. Is it easier to tell somebody about the house you are building, or is it easier to just build the house and then show them? Of course, the second is easier, because the other person will be able to visualize the same thing that you are.

In essence, models are very useful because they help to eliminate confusion and simplify ideas - helping scientists to better communicate their findings and ideas, and helping students to learn simplified concepts that can be build on later.

Learning is often easier if you can see a picture of what you're trying to learn about. Think about a layer cake. You would know a lot more about the cake if you could see a picture of what the cake looks like when it's cut. With the earth's layers, we understand better when we can see a picture of how they might look.

We can't actually get into the deeper parts of the Earth to see what is there, so we need models to analyze the data that we can get from earthquake waves.

Models are useful to scientists since there are things which we cannot study in nature either because they are very rare, or because we cannot directly see or measure them. In the case of composition layers of the Earth, you can imagine it would be very hard to see and measure these things directly. Since we cannot actually cut the Earth in half to see what is going on in the different layers, we can use models to better understand what might be happening.

Usually when scientists make models they think about the things that they do see and observe, and try to come up with explanations for why this happens. Once they have the explanation they try to come up with the model - a theoretical explanation for what they saw. However, models are just one explanation for why something happens. Another model may also give the same answer! This is why there is so much research into models. Scientists are always finding improvements which better reflect new discoveries.