Why after importing Standard Assets my Visual Studio Solution has 3 projects ?

Hi! I'm new with Visual Studio and I've been wondering why are there 3 projects inside my solution. They appeared after I imported some Standard Assets to my game.

I'll add an image so you can understand better what I'm talking about:

So my questions are:

1. What is each project for? I'd like to understand what are Unity and Visual Studio doing behind the scenes to make this 3 different projects part of the solution.

2. Can I add more projects to my solution to structure the code and still make everything work as in one single project?
   

3. When it could be a good idea to divide code in different projects inside a solution?

I've tried searching online but I could not find anything about how Visual Studio solutions/projects relate here to Unity scripts.