Economics make assumptions in order to predict what future trends might be. They make these assumptions based on historical data as well as formulas and calculations that have been proven in the past to work.
Why Do Economists Make Assumptions?
It is a normal thing for economists to make assumptions. The unpredictability of an economy is probably the main factor why economists do these things. As you may already know, economics deals with the production and distribution of products and services. This also involves the consumption of these products which affects the supply and demand in the market.
Answer
Economic model does not describe reality exactly. An economic model is a framework of a real thing or a theoretical representation of logic's illustrating the complex process of designing the expected outcome.