What’s the relationship between pressure and volume

Pressure and volume are inversely related. You can derive this from Boyle's formula P1V1=P2V2. Because pressure and volume are on the same side, as one increases, the other decreases.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.