Models are typically used to represent complex systems and to determine answers to questions about systems that can vary in complexity.
When learning about elements in basic chemistry, for example, a very simple model of an atom showing electrons, protons and neutrons can be used to show how atoms combine to form elements. This same simple model of an atom cannot be used to show reactions occurring in a super collider for a Nuclear Physicist.
A basic class on Astronomy may use a simple model of the solar system to explain the periods of the various planets orbiting the sun and the satellites orbiting the planets. This same model would be useless when making plans to send a satellite to Venus.
In the same way a Sociologist may use a population model when studying the causes of crime that is completely different than one used by an Anthropologist studying immigration trends.
Models will typically leave out certain physical or other aspects that are not needed for the purpose of the model, to make understanding simpler. Things like exact scale of size or distance between objects, weight, or even influence of other systems, may not be important when working with one subject (or science) but may be very important when working with another. Think of it this way; If someone asked you to design a model of the earth, you would need to know what they want to see in their model. Do they want a model showing land masses and major bodies of water? Do they want a model showing the core, mantle, crust and the details of what's inside the earth? Do they want a model showing examples of people and animals and plants that appear at different places on earth? Do you need to include cities, towns, roads, clouds, jet streams, bus and train routes, bird migration routes, etc.? You could probably build an unlimited number of models that could be used for different purposes and different sciences.
I hope this helps.