Why put asp.net MVC 5 model to another project?

I read some code of asp.net MVC 5 recently.

Found that they put the model into another class library project.

Then reference that lib from another asp.net MVC 5 project within the same solution.

But I can't understand why they do that.

Why don't they put the model into the models folder of MVC project?

Instead separate them?

I do survey over the Internet but can not found concrete view point to support the way they did.

Could somebody explain the pros and cons for me?

Thanks!