Balance the equation
__Cu+__Ag(NO3)——__Cu(NO3)2+__Ag

Balanced chemical equation for above reaction :

This is a displacement reaction, where copper (Cu) being morereactive than Silver (Ag) displaces Silver from its compound " Silver Nitrate " to form " Copper Nitrate " and silver .

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.