An optimist sees the glass half full. A pessimist sees the glass half empty.
A chemist sees the glass completely full - half with a gas and half with a ...

plasma

gas

solid

liquid

I think he sees gas and liquid

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.