Q:

Why organize projects inside a src folder? 

First of all, I would like to make it clear that I know that the src folder is not required.  Indeed, one can simply create manually one directory for the project, make one valid project.json and one Startup class and everything should work fine even without Visual Studio. 
My point is the following: when we create a new project using ASP.NET 5 from Visual Studio 2015 it creates a solution and inside the solution's folder it creates one src folder.  Within this folder all projects are created. 
Now why would anyone want to make one src folder inside the solution folder?  Why not putting the projects direct onto the solution folder?  Is there any advange on the organization of the project to put the projects inside a src folder?  Why VS does that now? 

A:

You usually have more files inside your project that are not source code related like:

README.md
CONTRIBUTING.md
.gitignore
LICENSE
Build scripts
Docs and tools folders, etc. 

And a tons of others files that depend on your configuration.  So, if you have a src folder, you don't need to mix your source code files with those. 

A:

because you may have your code in src folder (class libraries, etc), test in your test folder, documentation in a documetation folder
in global.json you specify which folders does roslyn pick for compiling.